SB 608 
.C885 
D6 
Copy 1 



^ITED STATES DEPARTMENT OF AGRICULTURE 
BULLETIN No. 879 

Contribution from tlie Bureau of Plant Industry 
WM. A. TAYLOR, Chief 



Washington, D. C. 



PROFESSIONAL PAPER 



November 15, 1920 



THE MOSAIC DISEASE OF 
CyCURBITS 

By 

S. P. DOOLITTLE, Assistant Pathologist 

OflBce of Cotton, Truck, and Forage Crop 

Disease Investigations 



CONTENTS 

Page 

Scope of tlie Investigations 1 

Tlie Mosaic Disease 1 

Cause of tJie Disease 18 

Infectious Nature of tlie Disease 29 

Mosaic Transmission 40 

Method of Overwintering 47 

Control Measures - 63 

Summary 64 

Literature Cited 68 




WASHINGTON 
GOVERNMENT PRINTING OFFICE 

1920 



P^> 



VP^ 



THE INVESTIGATIONS of the cucumber mosaic reported 
in this paper were begun by the writer in 191 4 at Hamilton, 
Mich., while a graduate student at the Michigan Agricultural 
College. In 1915 and 1916 they were continued at Big Rapids, 
Mich., and m 1917-18 at Madison, Wis., under cooperative 
relations between the Bureau of Plant Industry of the United 
States Department of Agriculture, the experiment stations of 
Michigan and^ Wisconsin, and certain interested pickle growers, 
under the joint direction of Dr. E. A. Bessey, Dr. L. R. Jones, 
and Mr. W. W. Gilbert. Laboratory facilities and material 
were furnished jointly by the cooperating parties and land, 
tools, and labor by a local pickle c(jmpany. 

The writer wishes especially to thank Messrs. Bessiey, Jones, 
and Gilbert for assistance and supervision throughout the 
course of the work. 

This paper was presented to th<^ faculty of the University of 
Wisconsin in partial fulfilhnent of the requirements for the 
degree of doctor of philosophy. 



n. 



4€^ 



Cf 



^ 



UNITED STATES DEPARTMENT OF AGRICULTURE 







^ BULLETIN No. 879 i 



Contribution from the Bureau of Plant Industry 
WM. A. TAYLOR, Chief 




S>Js^<^^u 



Washington, D. C. 



PROFESSIONAL PAPER 



November 15, 1920 



By S. 



THE MOSAIC DISEASE OF CUCURBITS. 

p. DooLiTTLE, Assistant Pathologist, Office of Cotton, Truck, and Forage Crop 
Disease Investigations. 



CONTENTS. 



Page. 

Scope of the investigations 1 

The mosnie disease 1 

Cause of the disease 18 

Infectious nature of the disease 29 

Mosaic transmission 40 



Page. 

M ethod of overwintering 47 

Control measures 63 

Summary 64 

Literature cited 68 



SCOPE OF THE INVESTIGATIONS. 

DURING the last four years the more important diseases of the 
cucumber have been studied by various workers in coopera- 
tion with the Bureau of Plant Industry- of the Ignited States Depart- 
ment of Agricidture. One of the most significant facts developed 
in the early work was the occurrence of a mosaic disease, heretofore 
practically unrecognized, which is probably at the present time the 
most widespread and destructive disease of cucumbers. The disease, 
however, is not confined to the cucumber but affects to a greater or 
less extent most of the cucurbits grown in this country. 

The present bidletin deals with the nature, transmission, and 
overwintering of the disease in relation to cucurbits, llie greater 
portion of the work has been done with the cucumber as grown for 
pickling purposes, in an attempt to develop field control measures. 

THE MOSAIC DISEASE. 

HISTORY. 

Although mosaic has been reported in the field and greenhouse for 
some years, it is only recently that the disease has received detailed 
attention. Selby (26)' in Ohio in 1902 and Stone (29) in Massa- 
chusetts in 1909 recorded a mosaic disease on the leaves of green- 
house cucumbers, and Clinton (8) in 1908 noted a chlorosis of musk- 
melon leaves in Connecticut. It is not certain, however, that all 



> The serial numbers in parentheses refer to " Literature cited " at the end of the bulletin. 
185118°— 20 1 



2 HULLKTIN H7l>, U. S. DI•:^AUT^ll:^'T OF AURICULTUIU:. 

those wrro truo mosaic, as the discMiso was not proved to !)(> infectious 
in any ease and no mention was made of symptoms on tin' fruit. 
Selhy (27) stated in 1910 that cucumber mosaic was transmitted like 
flinl on tol>acco. 'Hiere is no early record of the disease in the field, 
l)ul i( is certain that it liad caused serious losses in the ( 'entral States 
for some time prior to 1914, and from evidence obtained from pickle 
men in the vicinity of Chicago it is practically certain that the mosaic 
had been a serious disease in that section for 10 to 15 years aiul was 
responsible for forcing at least one pickle company out of business. 
Ruggles and Stakman (25) in 1911 reported the mosaic as occurring 
in Miiniesota and used the name wart disease in describing it. Coons 
(10) in 1915 reported that this disease was then causing serious losses 
in the field in Michigan and had been present in some localities for 
seven years, lie described the chief symptoms and stated that the 
j)rogress of the disease indicated that it was infectious. 

The first proof of the infectious nature of the disease and the 
methods by which it is transmitted was presented in the papers of 
(Jilbert (14), Jagger (17), and Doolittle (11) in 1916. Jagger (18) 
later reported the appearance of another type of cucumber mosaic, 
afTecting oidy the leaves of the plant, and in a furtlier paper (19) 
mentioned a third type as occurring on Summer Crookneck s(juash. 

Stakman and Tolaas (28) mentioned luibbin or wart disease of 
cucumber as an infectious disease occurring in both the field and 
the greenhouse in Minnesota. McCIintock (22) in 1916 observed 
possible cases of transmission of tlie disease through the seed and 
described its occurrence in tlu« district near Noi-folk, Va. Clinton (9) 
also gave data as to the severity of the disease in Connecticut. 

Freiberg (13) in 1917 reported the disease on various cucurbits in 
Missouri and described inoculation experiments. Jagger (20) in 
191S also reported numerous cross-inoculations from cucund)er lo 
other cucurbits and showed that cucumber mosaic could be trans- 
mitted to plants outside the Cucurbitaceie. 

These papers include all the available nuiterial on cucund)er 
mosaic to the present, with the exception of an abstract by Doolittle 
and Gilbert (12) which dealt with certain phases of the work hero 
considered in greater detail. 

GEOGKAPHICAL DISTKIBHTION. 

The general occurrence of cucurbit mosaic in the field was not 
recognized until after these studies were l)egun, but investigation 
has shown that it is widely distributed on Cucurbitacctv throughout 
the Cnited States. Doubtless tlie extensive ciJtivation of cucumbers 
makes it seem more common on that crop, i)ut it may occur on prac- 
tically all cucurbits in localities where it is found on cucumbers. 

The disease is probably most widespread and s(>rious in Wisconsin, 
Michigan, Indiana, northern Illinois, and on Long Lshuul. It is ro- 



THE MOSAIC DISEASE OF CUCURBITS. 6 

ported to have caused serious losses in Illinois as early as 1908 and 
was present about as early in Michigan and Wisconsin. The center 
of the pickle-growing industry is located in these States, wliich 
devote a largo acreage to the crop. Hero tlio disease is present to 
some extent in almost all localities, and in most cases there is infec- 
tion eveiy year. 

The disease occurs commonly in New York and is becoming serious 
in the seed-growing districts of Ohio and Iowa. It has been found 
quite prevalent in the fields around Rocky Ford and Greeley, Colo., 
causes severe losses in the trucking region about Norfolk, Va., and 
does much injury in many sections of Florida and Louisiana. Recent 
inspections of the principal city markets of the Western and Pacific 
Coast States have shown the mosaic disease to be present in practi- 
cally all of them, and in many cases it was very prevalent. It is 
also reported in the field from Maine, Massachusetts, Minnesota, Ver- 
mont, Connecticut, West Virginia, New Jersey, Pennsylvania, 
Georgia, Texas, California, Nebraska, and Ontario, Canada. 

In the greenhouse it has caused serious injur^^ for several 
years in Michigan, Wisconsin, Illinois, and New York and has also 
been reported from Mississippi, Ohio, Indiana, Minnesota, Pennsyl- 
vania, Louisiana, and Kansas. Up to the present the disease has 
been found only in the United States and Canada, but the fact that 
it has not been reported from other parts of the world may be due 
to its not having been extensively studied elsewhere. 

ECONOMIC IMPORTANCE. 

Cucurbit mosaic is the most serious disease of cucumbers in the 
Middle West, and yearly it increases in severity in districts where 
this crop is grown on a large scale. The disease is not confined to 
the cucumber, but causes considerable loss on the muskmelon and 
to a less extent on squash and pumpkin. 

Tlie greater losses on the cucumber are due partly to the special 
severity of the disease on the fruits of this host, but more especially 
to the fact that the acreage of cucumbers far exceeds that of any 
other cucurbit. The increase in the pickling and trucking industries 
in certain sections has localized much of this crop in restricted areas, 
and thus furnished conditions very favorable to the spread of such 
a higlily infectious disease. The other cultivated cucurbits ordi- 
narily are grown on a smaller scale, and thus the loss is distributed 
among many small growers, so that it is less noticeable. Cucumber 
plants infected with mosaic are practically worthless, OA\dng to the 
great reduction in yield and to the fact that the fruits produced 
are so mottled and deformed that they are usually refused by pick- 
ling companies and are of little value for market purposes. The 
disease spreads very rapidly, and many fields may become 50 to 75 
per cent diseased almost before picking has begun. As a result of 



4 BlLLl/riN 87!>, U. S. UEPAUTMENT i)V AGRICULTURE. 

tlio losses from iiiosuic, inany <;ro\v('rs, osjx'cially tliosc in certain 
sections of the Middle West, have ceased to grow cucuiiibei-s, despite 
increased prices (PI. I, ('). 

In the case of forcin<;j cucuinhei-s, growers occasionally lose an 
entire croj), because, in addition to the reduced yield and deformed 
fniit, mosaic often causes the sudden wilting and death of the plants 
under glass. An additional factor in the loss occasioned by mosaic 
is the fact that all'ected tabic stock sells for about one-fourth the 
price obtained for sound fruits, owing entirely to the effect on their 
ap|>earance. 

Of the other cidtivated cucurbits squash and muskmelon seem 
most susceptible to the mosaic, but neither of these plants is injured 
as severely as the cucumber. 'Hie muskmelon vines are stunted 
and bear only a few fruits, which arc inferior in qujility but only 
occasionally show mosaic symptoms. Squashes are similarly affected 
and in addition sometimes have warty and deformed fruits, the 
Summer Crookneck variety being most often and most severely 
attacked. The disease is less common and of minor importance on 
the pumi>kins, gourds, and ornamental cucurbits, which are of less 
commercial value. 

CUCURBIT HOSTS. 

Most of the species of the family C'ucurbitacea^ appear to be sus- 
ceptible to mosaic. Field o])servations have proved that most 
cultivated species are commonly affected, and that the disease also 
occurs on ^^ild species. The host range has been further extended 
by cross-inoculation experiments which included most genera and 
species of cucurbits found in this country and many varieties of 
the commoner cidtivated species. 

Most of the field inoculation tests were made during 1916 and 
1917. All inoculated plants were kept under insect-proof cages, 
and reciprocal inoculations were made in most cases. The various 
species were usually inoculated in stems or leaves with the expressed 
juices of mosaic cucumber plants, but many successful uioculations 
were made between other species, leaving no doubt that the disease 
is identical on the various cucurbit hosts. From 10 to 50 successful 
inoculations have been made with all the species noted below. 

In the case of cucumber, Cucuniis sath^us L.. 15 varieties were 
inoculated successfully, and all seemed equally susceptible. Mosaic 
was also produced on the West Indian gherkin. C. nnguria L.. on a 
cl(jsely related variety, C. grossulariatforntis Ilort., and on C. nietul- 
lijerus Mey. 

Inoculations on 11 A'arieties of muskmelon, Cucuims mrlo L., 
have all produced the disease, including the pomegranate melon, 
C. melo var. dudaim Naudin. Seven varieties of squash and four 
varieties of pum|)kin, Curiirhiid jxpo L., C. itutschata Duchesne, 
and C. ifi/ijcliiKi Duchesne, have also been infected. 



THE MOSAIC DISEASE OF CUCURBITS. 

On the gourds infection occurred on 12 varieties, including Cucur- 
hita pepo L., Luffa cylindrica Roehm, L. acutangula Roxbg., Lagenaria 
vulgaris Sor., and L. leucantlia Rusby. 

Mosaic has also been produced by inoculation on the following 
species: Momordica involucrata E. Meyer, M. cliarantia L., Ecballium 
elaterium A. Rich., Benincasa Mspida Cog., and Trichosanthes 
anguina L. 

The wild species, Micrampelis lohata (Michx.) Greene, is common 
in many parts of the country both as an ornamental vine and grow- 
ing wild and is often diseased. The identity of this disease with 
that on the cucumber as well as with that which occurs on the less 
common wild species, Sicyos angulatus L., has been proved by 
repeated inoculation. 

From data so far collected, the watermelon, citron, and other 
species of Citrullus seem partially resistant to the disease. No 
successful inoculations have been made by the writer on any plants 
of this genus with the exception of the green-seeded citron, Citrullus 
vulgaris Schrad., and watermelons grown in the field have never 
been found infected although often intertwined with mosaic-diseased 
cucurbits. Jagger (19) reported havmg obtamed a single case of 
mosaic on watermelon by inoculation, but was not able to repeat 
such infection. He also noted the disease on the green-seeded 
citron, but not on the red-seeded variety. Freiberg (13), at the 
Missouri Botanical Gardens, also reported mosaic on the citron and 
showed photographs of the disease. 

CROSS-INOCULATIONS TO OTHER PLANTS. 

INOCULATIONS FROM MOSAIC CUCUMBERS TO PLANTS OF OTHER FAMILIES. 

Certain field observations have at times suggested that cucurbit 
mosaic might possibly be transmitted to plants of other families. 
An example of such a case occurred at Madison, Wis., in 1916. 
A few plants of Martynia louisiana Mill, appeared in one of the 
cucumber plats, the seed having been accidentally included with 
tli^t of the cucumber. A large number of the cucumber plants 
became infected with mosaic early in the season, and a few of the 
martynia plants also developed a disease of the m.osaic tj^pe later 
in the summer, the circumstances suggesting a possible transfer- 
ence of the infection from the cucumber. Inoculations were made 
from the mosaic martynia to healthy cucumber plants and also 
from mosaic cucumber to healthy martynia, but all the inoculations 
gave negative results. 

Most of the inoculations from cucumber to plants outside the 
Cucurbitacefp, however, have been with plants known to be subject 
to infectious mosaic diseases, such as tomato, tobacco, bean, petunia, 
and pokeweed {Phytolacca decandra L.), although a few inoculations 
have been made on plants subject to unknown chlorotic diseases. 



6 



BULLKTIN 879, U. S. DEPARTMENT OF AGRICULTURE. 



The motliods used in tliis iiloculation work were very similar to 
those used in the other (•ueum])er-mosiue studies. Wliere fiehl inoc- 
ulations were made, the i)hints were protected with insect-proof 
ca«;es wlierever possihle, and iti tho g^reenhouso all plants used in 
surli inocuhvtions were kept isolated from other mosaic experiments. 
The inoculum was prepared in much the same way as in other 
cucumher-mosaic inoculations, either the expressed juice or crushed 
tissues of mosaic ])lants heinj? used. Wiiere the juice of a mosaic 
plant was employed, tiie parts of the plant used as inoculum were 
passed throu{j:h a sterilized food chopper and tho juice expressed and 
filtered tlirou<;h filter paper. Inoculations were made at from three 
to five points in each of the younger leaves, a drop of the exjiressed 
juice heinj; pricked into the leaf with a sterile needle. The stem 
usually was inoculated at one or more points, a slight incision being 
made with ii sterile scalpel carrying a drop of tlH> inoculum. 

Taiuk I. - Rtcurd itj inncuUitiom from viosair-diseascd cucumber plants to plants oj 

other families. 



Dateinoai- 
lated. 



Aug. 
Mar. 



Mav 
Oct. 
Nov. 
Aug. 
Aug. 



4,1916 
_ 11.1917 
Apr. 27,l"Jl7 
Mav 8, 1917 
20,1917 
1,1917 
6,1917 
4,191G 
,. 18,1916 
Feb. 2.3,1917 
Feb. 30,1917 
Apr. .5,1917 
Oct. 8,1917 
Dec. 8,1917 
Aug. 4,19Ki 

Do 

Feb. 1.5,1917 
Mar. 24,1917 
Sept. 7,1916 
Aug. 15,1916 
Aug. 16,1915 

Mar. 22,1916 
Aug. 1,1917 
Aug. 5,1917 



Plant inoculated. 



Tobacco 

do 

do 

do 

do 

do 

do 

Tomato 

do 

do 

do 

...do 

...do 

...do 

Wax bean 

I-inia bean 

Red kidney beau , 

do 

Martynia louixiana 

Potato 

Pokeweed (Phytolacca dt- 

randra.) 

Petunia 

A mbroxia trifida 

....do 



Preparation ofinoculum. 



Expresi^ed juices 

do 

do 

do 

Cru.shed leaf tis-^ues . 

do 

E.\ pressed juices 

do 

....do 

....do 

....do 

Crashed leaf tis.sues. 

....do 

E xpressed j uices 

do 



Number 
of plants 
inocu- 
lated. 



.do. 
.do. 
-do. 
.do. 
.do. 
-do. 

.do. 
.do. 
.do. 



Results. 



Number 

of mosaic 

planus 



Date last 
observed. 



Aug. 17,1916 
Mar. 26.1917 
Mav 28,1917 

Do. 
June 14,1917 
Oct. 30,1917 
Dec. 1,1917 
Aug. 17,1916 
Aug. 31,1916 
Mar. 20,1917 
2, 1917 
8.1917 

19. 1917 

12. 1918 
Aug. 17,1916 

Do. 
Mar. 10,1917 
Apr. 12,1917 
t^ept. 23, 1916 

Do. 
Sept. 30, 1915 

Apr. 10,1916 

Sept. 1,1917 

Do. 



Mav 
May 
Nov. 
Jan. 



Where the crushed tissues of tho mosaic i>lant were used, a frag- 
ment of one of the young leaves was crushed in a sterile dish with 
sterile instruments and small portions of this crushed material in- 
serted in slight in(isi(»iis made at one or two points in tho stem of 
the ])lant hioculated aiul other ])i('ces sometimes pricked into tho 
young leaves. 

The results of all the inoculations from mosaic cucumber to ])lantfi 
outside the cucurbits have been negative,' as shown by Table I. 

' Rwent experiment? (1919) indicate that cucumlK>r mosaic may be tninsmltted to ilarifnia louitiana 
by mcims of uphills lukcii from ni(is;ilc cuciimbrr plants. Inoculations from martynia pliujts infected In 
this miuiucr have also produced tho disease oa tho cucumber. 



THE MOSAIC DISEASE OF CUCURBITS. 7 

Jagger (20), however, states that the white-pickle type of mosaic 
disease, which is the one studied by the writer, has been transmitted 
to one of the Lobeliaceae {Lobelia erinus L. var. gracilis) and to one 
of the Compositse {Helianthus dehilis Nutt.)- It is thus evident that 
in some cases the disease may pass from the cucumber to hosts out- 
side the Cucurbitaceae. 

Table II. — Record of inoculations of cucumber plants with infectious mosaic diseases 
of noncucurbitaceous plants and with suspected mosaic plants of other families. 

Pl.vnts Having Mosaic Diseases Known to be Infectious Used as Source of Inoculum. 



Date inocu- 
lated. 



All jr. 

Sept. 

Dec. 

Feb. 

Aup. 

Sept. 

Feb. 

Feb. 

Dec. 

Feb. 

Dec. 

Aug. 

Feb. 

Mar. 

May 

Nov. 

Jan. 

Sept. 

Sept. 

July 

Aug. 

Mar. 

Sept. 

Sept. 

Aug. 

Sept. 



25. 1915 
S,1915 

21,1915 

4,1916 
19,1916 

7,1916 
18, 1917 
20,1917 

2,1917 
15,1918 
21,1915 
29,1916 
17,1917 
30, 1917 
20,1917 
10,1917 
11,1918 

:i, 1915 
10,1915 

15. 1916 
30, 1916 

5,1918 
3.1915 
in, 1915 
23,1916 
3, 1915 



Sept. 10,1915 
Aug. 23,1916 
Aug. 30,1916 
Mar. 19,1916 
Apr. 1,1916 
Sept. 3,1915 

Sept. 10, 1915 
July 20,1916 
Sept. lO; 1915 

Aug. 30,1916 



Plant used as source of 
inoculum. 



Mosaic tomato. 

....do 

do 

do 

do 

do 

do 

do 

do 

do 



Mosaic tobacco 

do 

do 

do 

do 

do 

do 

Mosaic bean 

do 

do 

do 

do 

Mosaic potato 

do 

do 

Mosaic pokeweed (Phyto- 
lacca aecandra). 

.do 

.do 

.do 



Mosaic petunia 

do 

Peach with yellows (leaves 

and twigs). 

do.... 

do 

Little peach leaves and 

twigs. 
do 



Preparation of inoculum. 



Expressed juices 

do 

do 

do 

Crushed leaf tissues . 

Expressed juices 

Crushed leaf tissues. 

Expressed juices 

do 

Crushed lea f tissues . 

Expressed juices 

do 

do 

do 

do 

Crushed leaf tissues . 

Expressed juices 

do 



.do. 
.do. 
.do. 
.do. 
-do. 
.do. 
.do. 
.do. 



do 

do 

Crushed leaf tissues. 

Expressed j uices 

do 

do 



.do. 
-do. 
-do. 

.do. 



Number 
of plants 
inocu- 
lated. 



Results. 



Number 
of mosaic 

plants. 



Date last 
observed. 



Mar, 
Jan. 
Mar, 
Jan. 



Sept. 10,1915 
Sept. 20, 1915 
Jan. 22,1916 
Feb. 27,1916 
Sept. 5,1916 
Sept. 23, 1916 
Mar. 3, 1917 
21,1917 
16,1918 
1,1918 
22, 1916 
Sept. 23, 1916 
Mar. 18,1917 
Apr. 20,1917 
June 14,1917 
Dec. 1, 1917 
Jan. 30,1918 
Sept. 20, 1915 

Do. 
Aug. 17,1916 
Sept. 23, 1916 
Mar. 23,1918 
Sept. 20, 1915 
Sept. 23, 1916 

Do. 
Sept. 20, 1915 

Do. 
Sept. 15,1916 
Sept. 23, 1916 
Apr. 1,1916 
Apr. 22,1916 
Sept. 20, 1915 

Do. 
Aug. 17,1916 
Sept. 20, 1915 

Sept. 23, 1916 



Plants HA^^NG Diseases of the Mosaic Type of Suspected Infectious Natube Used as Source 

OF Inoculum. 



Sept. 10, 1915 

.July 20,1915 
.\nfi;. 2.5,1916 
Sept. 10, 1916 

July 20,1916 
Sept. 7,1916 

Aug. 10,1917 
Sept. 8,1916 

Aug. 18,1917 

Aug. 30,1917 
Aug. 30,1916 



Mosaic mi\kweQA(.Asclepias 
syriaca). 
do 
do, 



Mosaic red clover ( Trifo- 

lium jrratense). 

do 

Mosaic mart ynia {Afartynia 

louisiana) 

do 

Mosaic pepper {Capsicum 

annuum). 
Mosaic ragweed {Ambrosia 

iTifida). 

do 

Mosaic sumac {Rhus ty- 

phina). 



Expressed juices. 



.do. 
-do. 



.do. 



.do. 
-do. 



Crushed leaf tissues . 
E.xpressed juices 



.do. 



Crushed leaf tissues. 
E xpressed j uices 



9 





5 
7 
10 







5 
8 






6 
3 






7 





8 
8 







Sept. 20,1915 



Aug. 
Sept. 
Sept. 

Aug. 
Sept. 

Sept. 
Sept. 



17, 1916 
23,1916 
20, 1916 

17, 1916 

23. 1916 

5, 1917 

23. 1917 



Sept. 12, 1917 



Sept. 
Sept. 



24, 1917 
23, 1916 



8 BULLKTIN 87f), U. S. I)i:rAi;TMKNT OF AtiUlCUl/rrilK. 

INOCULATIONS THOU MOSAIC PI-ANTS OF OTHKU FAMILIES TO CUCUMBER. 

Attempts have nlso boon made to transmit mosaic disoasos found 
on plants outsido tlio Cuciirhitacoay to tho oucumbor. A considorable 
numbor of inociihitions liavo boon mado from tomato, tobacco, bean, 
and other plants liaving infectious mosaic diseases and also from 
plants whicii showed diseases of the mosaic type but whose infectious 
nature was not proved, such as popper {Capsicum annuum L.), sumac 
{Rhus tijphina L.), milkweed {Asclepias syriaca L.), red clover 
{Tri folium priitcnse L.), and others. Inoculations in most cases 
were made with the expressed juice of tho mosaic plant in the manner 
already described. All the results of these inoculations have been 
negative, as shown in Ta])le II and there is as yet no evidence that 
any of the diseases on the hosts tried are identical with the mosaic 
disease on the cucurbits. 

MOSAIC SYMPTOMS. 

Tho profrross of tho mosaic disease in tlio plant is characterized by 
distinct morpliologic and physiologic chan.<;es, wliicli are most markod 
in tlio loaves and fruits. Those consist principally of an abnormal 
mosaic coloring, accompanied by certain malformations and dwaifmg 
of growth. Tho symptoms vary somowliat according to tlio ago of tlio 
plant and its vigor of growth at tho time of infection, but tho general 
characters aro tho same in all stages of growth. There is also a 
variation in tlio symptoms on tho different species of Cucurbitacoa? 
which aro susceptible to tho disease. Tho loaves of different species 
show many diverse forms of mosaic coloring, and the fruits may or 
may not bo afToctod, although usually they show markod s^iiiptoms 
soon after tho plant becomes diseased. Tho symptoms on tho cu- 
cumber have been described in greater detail, as it is the most im- 
portant host and has been most closely studicnl, but the general 
characteristics will apply to all otlior hosts except for the variations 
mentioned later. 

riAXT SYMI'TOMS. 

Siimpfoutft on (iffeclcd .see(Za>? 7.9.— Seedling cucuml^oi-s nns rarely 
affected with mosaic either in tho field or in tho greenhouse. The 
disease may aj)poar on seedlings under certain conditions, ]iow(>V(?r. 
as is shown by the fact that a very general and serious mosaic inf(H'- 
tion occurred on the first true leaves of cucuml)er seedlings at Big 
Rapids, Mich., in tho summer of lOlS. The seed was plantcMJ bite 
in August and when the plants came up the re^st of the iield was 
already rather badly infected with the disease. When such infection 
does occur, the first symptoms appear as a yellowing and wilting of 
tho cotylcHJons, accompanied b}^ a slight mottling of t!u> young 
loaves. A sudden chocking of growth soon follows and tho latci 
leaves remain small, wrinkled. mihI distort. m! iind have a distinct 



Bui. 879, U. S. Dept. of Agriculture. 



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4j.? 



^V^^ 



Mosaic on Young Cucumber Plants and a Badly Diseased Field. 

A, A single mosaic plant obtained from seed from mosaic plants in a test at Big Rapids, Mich., 1917; B, 
a young mosaic cucumber plant, showing the curling and savoycd ellect on the smaller leaves: Big 
Uapids, Mich., 1910; C, a cucumber field at Holland, Mich., on August 19, 191.5, showing severestunting 
and loss to the late-planted crop due to infection w hile the plants were small; theearly planted portion 
in the background was not so badly allected. Photographed by ^\. W. Gilbert. 



Bui. 879. U. S. Dept. of Ai:rirulture. 



Plate II. 




Mosaic Cucumber Plants. 

A , Mosair cuciimldT plant infcctwl whili' voiiiik, sliowlnR ilwarCmc and curling of the Iravos and shortcJi- 
iiiK of tlio st.in'.; (ir.-clcy, Colo., SoptcmlM-r, 1'.I17. rhot(i>:niplii'd by W. \V. (iillxTt. H. Cuciimhcr 
mosjiic frrnii iiiiMiilulion "with apliiils from ini>saic plants, the fruit svmplonis Im-Uik most marked: Hig 
JCaplds, Mich., I'.il... rtiotov;raphi"il l.y \V. \V. (iillx-rt. r, .Mosaic plants in late stanc showing death 
of till' l)ii.sal leaves, whitening of the main stem, uud the dwurliug and curling of the tij) loaves. Photo- 
graphed by M. \V. Gardner, September, lUlO. 



THE MOSAIC DISEASE OF CUCURBITS. 9 

mottling of yellowish green. Such plants seldom produce fruit and 
are usually short lived, rarely reaching a length of more than 1 2 inches. 

Symptoms on 'plants infected wTien young. — The period of most 
general infection begins when the plants are about 6 weeks old and 
growing vigorously. At this time they have from six to eight leaves 
and are commencing to run. The first symptoms in all cases appear 
in the young leaves which are still in the process of development and 
usually are not easily recognized by an untrained observer. The 
youngest of the loaves develop small greenish yellow areas, often not 
more than a millimeter or two in diameter, occasionally circular, but 
more often limited in outline by the smaller veins of the leaf. These 
spots are slightly more translucent than the remainder of the leaf and 
are often scarcely visible except by transmitted light. Less commonly 
the normal green of the leaf changes to a peculiar yellow at the tip 
and the sharply defined yellow areas do not appear. Accompanying 
these symptoms there is a gradual downward curling of the edges of 
the leaf, and the surface presents a finely wrinkled appearance, the 
tissue between the small veins becoming sUghtly raised so as to form 
minute convex surfaces (PI. I, B.) Following these early symptoms, 
the gross mosaic characters develop rapidly, and the leaves become 
strikingly mottled with green and yellow. Such leaves are wrinkled 
and savoyed in appearance and may be somewhat distorted and 
curled (PI. II, A). 

All growth subsequent to infection is mucn dwarfed, the stem 
intornodes are shortened, the leaves attain only about one-half nor- 
mal size, and the petioles are reduced in length. As the plant be- 
comes older the wrinkled and savoyed character of the leaf is more 
marked, but the motthng is generally less conspicuous. 

Plants infected at this stage blossom sparingly and set few fruits. 
Thoy send out few runners and have a bunched and bushy habit of 
growth, with the leaves Ijmg close to the ground in a rosettelike 
clump. 

Siimptom,s on plants infected when nearly mature. — In the case of 
large plants the general symptoms are similar to those described 
above. The first signs of the disease appear in the youngest leaves, 
where the yellowing and curling before noted are the usual indica- 
tions of infection, while all the older leaves appear normal and may 
remain so for some time. In some cases, however, the youngest 
Ion ves turn yellow and wilt, their edges becoming brown and withered, 
and all the leaves of a shoot may tlius wilt within a few days. Ordi- 
narily, however, only three or four leaves at the growing point are 
afi"ectod in this way, and the runner soon sends out a new shoot just 
b;u^k of the withered tip, which shows the ordinary mosaic sj'mptoms. 
Symptoms may also appear very early on the young fruits, and in the 
first stages of the disease they are often more marked than those which 
occur on the leaves (PI. II, B). Frequently the most pronounced 



10 Bl'LLI-TIN 879, U. S. DKPAKT>rKNT OF AGRICULTURE. 

leaf syiiij)t(>iiis app(»ar on new swoudary shoots put out from tli(^ 
axils of the leaves near tho haso of tho ])lants. Tli<» wilt symptoms 
are often noted on tho cucumber and pumpkin and occasionally on 
the Summer C'looknix k scjuasli and muskmelon 

In till* t,Teenhouso tliis typo of icsponso is much more intens<», 
and it is conmion to have tho vine suddenly develop a yollowinnj and 
wiltini;, wliieh extends to the entire plant within two to three days, 
the older leaves usually sliowini; yellowinu; hut wiltinj; very rapidly. 
This wilting and death of the plant occur only occasionally in the 
fi(»ld and never prot^ressso rapidly there as they do in tho <;re(^nlious»v 

Aside from these diirerences, the older plants in the field show the 
same symptoms as plants infected at an earlier stage in their devel- 
opment. The dwarfing and wrinkling of the younger leaves are very 
])ronounced and the mottling is of the same tyj)e as that on younger 
plants. The shortening of the stem internodes and the petioles 
gives the ruimers a fhittenod appearance, and the leaves lie close 
to the ground. f 

The older leaves of mature plants are later more severely affected, 
and the greater portion gradually die off. Portions of the leaf turn 
yellow, those at the base of the stem being affected first. This may 
occur as a gradual yellowing of the entire leaf, or more characteris- 
tically as a V-shaped yellowed area along one of the large lateral 
veins of one of the basal lobes. The tissues of these j'ellowed portions 
rapidly turn brown and die, l)eginning at the edges in the angle formed i 
by the lobes and working toward the base of the lateral veins. (PI. ' 
III.) Tlie entire leaf eventually becomes yellow, this yellowing grad- 
ually extending to the leaves farther up the stem. Such leaves 
finally wither and die, leaving the basal portion of the stem bare. 
Tliis is a characteristic symj)tom in late stages of the disease, and by 
the end of the season many plants show 1 to 3 or more feet of bare 
stem with the dead leaves still attached, but terminating in a cluster 
of dwarfed, dark-green, mottled, and ^v^inkled leaves wliich lie 
close to the ground. (PI. II, C.) 

LEAF SYMPTOMS. 

The pattern of coloring which the leaves of mosaic plants assume 
varies with the species, the age of the plant at the time of infection. 
and the stage of development of the individual leaves. 

Leaf ftymptoinfi on cneunihers. — On the (•ucuml>er (Cunimi'i sativus) 
the mosaic mottling is usually most pronounced and tyjiical on the 
young leaves of plants which are growing rapidly (PI. IV, A). Tho 
green portions of these leaves are very dark in color and are raised 
slightly ai)ove the surrounding surface, thus giving the leaf its savoyed 
appearance. In most cases the green areas predominate, being 
intermingleil with spots of light greenish yellow. These yellow sj)ots 
are irre«riilar in outline and are limitetl bv the small veins (PI. IV, li). 



THE MOSAIC DISEASE OF CUCURBITS. 11 

In some cases tlie green and yellow areas are both small and about 
equal in number (PI. Ill) , being scattered irregularly over the leaf 
and sometimes coalescing to form larger areas, with the green portions 
rather sharply defined, owing to their raised character. More often, 
however, there are a few isolated patches of yellow from 3 to 5 milli- 
meters in diameter, the greater portion of the leaf being dark green 
with the typical savoyed character. On other leaves the yellowing 
may take the form of large blotches which tend to follow the large 
veins, the margins not being sharply defined but blending gradually 
into the green portions of the leaf. In such leaves the savoyed 
appearance is much less marked. 

In the case of older plants the wrinkling of the j^oung leaves is 
usually very noticeable, but there is less contrast in color. Tlie 
raised parts of the leaf are dark green, while the remainder is merely 
a lighter shade of green than that found in normal leaves^ the borders 
of these light spots not being sharply defined. In such plants the 
base of the leaf will occasionally develop a sharply contrasted mosaic 
mottling, while the tips of the leaves will remain normal in color 
for some time. 

In the older leaves the symptoms are somewhat different in nature. 
The savoyed character does not appear in many cases, and the yellow- 
ing tends to include the entire leaf in a short time. In leaves wliich 
are nearly full grown the yellowing very often extends along the net- 
work of the larger veins of one of the basal lobes. The portions of 
the leaf inclosed within these veins may remain green for some time 
and have a slightly convex appearance. A pronounced drooping 
or curling of the leaf often follows in the parts w^here such a mottling 
occurs, producing a peculiar wilted appearance in the, lobe of the 
leaf affected. 

In other cases certain portions of the leaf turn yellow and the re- 
mainder retains its green color for a considerable time. The yellow 
portions in such leaves often include a few small, isolated, dark-green 
areas, producing an odd, checkered appearance in these portions of 
the leaf. Tliis is quite distinct from the ordinary mosaic mottling 
in that the green tissues show no tendency to be elevated above the 
surrounding leaf surface. This type of yellowing occurs most com- 
monly in the angle formed by the junction of the large veins at the 
l)ase of the leaf, but also appears at the tips of the lobes. In such 
leaves the green portions retain their color for some time. One of 
the most common symptoms in the older leaves of mature plants 
commences as a yellowing which appears along the large lateral 
veins of one of the basal lobes, extending their entire length. These 
yellow areas are widest at the edge of the leaf and narrow toward the 
juncture of the veins, producing V-shaped areas which have already 
been described (PI. Ill) . 



12 BriJ.KTIN «70, IT. S. DF,rART>rKNT OF AGRICULTURE. 

Another syiMj)t()in on tlie oMi'i- lojivcs which is nioiv common in the 
grponhouso than in the fichl, consists of a yellowing which alTects 
nearly all the leaf at the same time. In such leaves the tissues 
ailjoining the larger veins retain their color longest and outline these 
veins against the yellow of the remainder of the leaf. These symp- 
toms are usually associated with the wilting type of tht disease. 

Leaf sijmptoms on inusk melons. — 'Hie leaf symptoms on the musk- 
melon {Cucumw melo) are very similar to those on the cucumber. 
The lii*st signs of the disease appear on the youngest leaves, which 
turn light yellow in color and develop a sharp downward curl. Later, 
all the younger leaves of the plant show typical mosaic characters 
(PI. V, B). The light yellow portions of the leaf are of irregular 
outline and about the same size and color as those on the young leaves 
of the cucumber. The dark areas are more definitely outlined on 
the muskmelon, and the leaves show a very pronounced curling. 
The characteristic symptoms found on the older leaves of the cucum- 
ber are much less common on the muskmelon. Tlie older leaves 
gradnallv become yellow, but show little or none of the tendency to 
die early which appears in mosaic cucumber leaves of the same age. 

Leaf symptoms on squashes. — In the case of the squash (Cucurbita 
spp.) the symptoms of the disease are much the same on all varieties. 
The 3'ounger loaves usually develop an extremely savoyed appear- 
ance, the darker parts of the leaf being much more definitely raised 
above the leaf surface than in the case of the cucumber. The light 
spots on the leaf are pale 3x4lowdsh green and tend to coalesce and 
form a few large blotches rather than the smaller and more numerous 
spots found on the cucund)er (PI. VI, A). The older leaves often 
become yellow and gradually' die off, as in the case of the cucumber, 
but in many plants these leaves will retain their normal color for 
some time, the symptoms of the disease being confined to the younger 
leaves. The most marked symptoms have been found on the Summer 
Crookueck and Cocozelle bush varieties {Cucurbita pepo Yar-condensa), 
those of the Hubbard type (Cucurbita maxima) showing the disease 
in a less extreme form. 

Leaf symptoms on pumpTcins. — On the pumpkin {Cucurbita pepo) 
the leaf symptoms are very nearly identical with those found on the 
squash. The younger leaves develop with the same extreme mottled 
and wrinkled appearance, the general color of the leaf being a lighter 
yellow than that of a normal plant. On the pumpkin, however, the 
older leaves very of ten show a rapid yellowing and wilting which corre- 
sponds very closely to that found on the cucumber in the gi(>cn house. 

Leaf s-ijvipfonis on oilier cuciirhits. — The general symptoms on the 
gourds and (»|Ium' ornamental cucurbiisi are very similar ((► those on 
the squash. The chi(>f points of difTerence occur in the patterns of 
the mottled Icaxcs. In the case of the (^ucurbita i;(»urds the surface 



Bui. 879, U. S. Dept. of Agriculture. 



PLATE III. 




AiloenLCoBallrn 



MOSAIC LEAVES, FRUITS, AND BRANCH OF CUCUMBER. 



Three fruits illustrating cominon types of mosaic effect; two leaves, the smaller showing pro- 
noimced mottling, comroon to yoiing leaves, the larger showing dying of older leaves on plants 
infected when nearly mature ; tip of plant with dwarfed and dying leaves. 



Bui. 879, U. S. Dept. of Agriculture. 



PLATE IV. 




Mosaic Cucumber Leaves. 

A , Small cucumber leaf, showing extreme mottling and wTinkling characteristic of a severe mosaic attack; 
Madison, Wis., 1919. B, Cucumber leaf, showing simple mottling often seen in the earlier stages of 
mosaic attack; Big Rapids, Mich., 1915. Photographed by W. W. Gilbert. 



THE MOSAIC DISEASE OF CUCURBITS. 13 

of the leaf is finely wrinkled and the yellow areas are very small but 
present in large numbers, so that the leaf appears to be covered with 
fine dots of yellow, which in some cases bear a close resemblance to 
insect injury. This type of mosaic symptoms is also common on the 
Luffa and Lagenaria gourds, the spots in the latter species being 
larger and very nearl}^ circular in form. (PI. V, A.) 

On the wild cucumber {MicrampeUs lobata) there is usually a 
marked variegation and deformity in the leaves of mosaic plants. 
In most cases the symptoms are similar to those on the cucumber 
with the exception that the lighter colored portions of the leaf pre- 
dominate. These are a light yellowish green with an occasional 
almost colorless area which is in sharp contrast to the surrounding 
tissue. The other parts of the leaf are a deep green and are raised 
above the surrounding surface. As a result the savoyed character is 
usually pronounced, and the leaves are deeply wrinkled and curled. 
There is also considerable dwarfing and deformity in mosaic leaves, 
the lobes being irregular in size and outline. In some leaves, how- 
ever, the difference in color in the various parts of the leaf is less 
marked, and in such cases the savoyed character does not appear, 
although the leaf may be distorted in shape. (See PL VII, A^ B, 
and C, for comparison.) 

While the above types of symptoms occur most commonly, there 
are often cases in which the greater part of the leaf remains almost a 
normal green but develops numerous small circular areas (5f light 
yellow which are similar to those noted on the Cucurbita gourds. 
Tliese yellow spots are about 1 millimeter in diameter and are slightly 
depressed. In other leaves we find a number of small, dark-green 
areas scattered over the surface, the center of each of them being 
composed of a small yellow spot similar to those above described. 
The green portion of these areas is raised, while the yellow centers 
are slightly sunken, producing a peculiar pitted appearance. 

The older leaves of mosaic plants gradually turn yellow, some of 
them shoMang the V-shaped yellow patches which occur on the older 
leaves of the cucumber. This yellowing of the leaves eventually 
affects most of the plant and is a distinguishing character of the later 
stages of the disease. 

The symptoms on the other wild host, Sicyos angulatus, are much 
like those on micrampelis. The mottling of the young leaves is 
seldom extreme, but the older leaves tend to yellow and die off rapidly, 
the dead leaves remaining attached to the stem as in tlie case of the 
cucumber. 

FRUIT SYMPTOMS. 

Finiif sympfoms on cucamhers. — The symptoms on the fruits of the 
cucumber are extremely marked in most cases and when present 
make the disease easily recognized. The stem end of the young 



14 lU'LLKTlN 87S), U. S. DKPARTMENT OF AGRICULTURK. 

fruit fii-st l)o<'om«'s mottlod with yellowish groen, this mottled char- 
artor giadiiiilly sjnoadiiij^ ovc^r tho oiitiro fruit. As tiiis pros^rcsses 
tliobod}' of tho fruit ordinarily hecomos a li<;^ht yellowish jrreen, intor- 
niinp;l(»d with spots of a much darker groon color. These dark por- 
tions are usually raised above the surrounding surface in such a way 
that they form wartlike projections and often produce more or less 
distortion of the fruit. 

Till* numhei- and size of these protuberances vary greatly, and we 
find many tj-pes of affected fruits (PI. VIII). In some cases, particu- 
larly in tho greenhouse, the mottled character is very marked, the 
fruit being covero<l with minghnl blotches t)f liglit and dark groen, but 
with little or no wart production or distortion of shape. Such fruits 
are common in the field, as also those in wliich the stem end is the 
only ])i»rt to show a well-defined mottling. 

More commonl3% however, the diwk-green portions of the surface 
are distinctly raised, tho projections varying from 1 millimeter up to 
2 centimeters in diameter and from 1 to millimet<MS in lieight. The 
general appearance of all such fruits is the same, but the minor char- 
acters are subject to great variation. In some cases tho warts are 
small, sharply outlined, isolated, and scattered over the surface in 
varying numbers (PI. VIII, A to C). On other fruits they are larger, 
coalesce to some extent, and form irregular raised patches, which are 
sometimes slightly elevated and do not greatly afl'ect tho shape of the 
fruit, while in still other specimens they are so large as to produce an 
extremely rough and iiTogular form (PI. VIII, D, E). In some cases 
theio may bo one or two largo warts, often at tho stem end, tlie 
remainder- of the f luit being yellowish white or mottled with yellowish 
green, but nearly as smooth as normal. 

In general, however, tho fruits are decidedly mottled in appearance 
and show swellings of aU sizes, some isolated and otliers merging 
into one another in such a way as to produce a very irregular form, 
the symptoms oiUm being well defined on fruits which are but an 
inch in length. Tho fruits of this typo have given rise to tho names 
"wart disease" and "nubbin," which have been applied to tho disease 
by growers in some parts of tho country. 

In tho later stages of tho disease the vinos occasionally produce 
fruits which are smooth, pale whitish green in color, and rather more 
blunt at the (»nds than noinial fruits of tlie same ago. In most cases 
those fruits are mottled with fine spots of yellowish green, and a fow 
dark-green projections appear here and there on the surface. (PI. 
\TI1.) Tluvse are usually small, but occasionally fruits are found 
which hnvo a single largo dark-green swelling near the stem end, 
producing a most unusual appearances. 

Those wiiite fruits are responsible for tlio older common namt^ of 
tlio disease, " white ])ickle," which was tho term applieil to tho trouble 



THE MOSAIC DISEASE OF CUCURBITS. 15 

by tho growers in Michigan and Wisconsin for some years. The 
more common greon and distorted fruits wore classed witli tho imper- 
fect types which are often produced under unfavorable «!oil or climatic 
conditions, as the name ''nubbin" in use in Minnesota indicates. 
All those names, however, neglect the mottled character of both 
leaves and fruits, which is the symptom most typical of the disease. 
Since the term mosaic had already been applied to a disease of 
tobacco and other plants in which the symptoms and general char- 
acter were much like those on the cucumber, it seemed best to adopt 
the same name in this case as being more descriptive of the real 
nature of the disease than those already in use l)y the growers. 

On vines which are old and stunted another type of fruit often 
occurs which is abnormally dark green, with little or no mottling. 
Such fruits have a smooth surface, l)ut are much distorted and swollen 
in appearance and are found only at the end of the season on 
vines which have nearly ceased to set fruit. Vines occasionally 
appear to recover from the effects of the disease and toward the end 
of the season produce fruits with no visible mosaic symptoms. 

Many mosaic fruits show a tendency to become russeted while 
still green, and in the case of the whitish fruits before described this 
tendency is very marked. As they turn yellow and ripen, the fruits 
which show less marked mosaic characters may be almost indis- 
tinguishable from those of healthy plants, but in general they retain a 
misshapen and dwarfed appearance. Seed is usually produced, but 
in the case of fruits set during the later stages of the disease, the seed 
is slow to mature and often small and shrunken. Mosaic fruits may 
also have a slightly bitter flavor, particularly those from plants 
which have had the. disease for some time, but this is not a common 
symptom. 

Fruit sym'ptoms on other cucurhits. — Although the leaves of all the 
cucurbits susceptible to mosaic show symptoms which have the same 
general character, the fruits in some cases show no evidence of the 
disease aside from a reduction in size. In general, however, the 
fruits of other mosaic cucurbits have the same mottled and warty 
appearance that is found on those of the cucumber, although these 
symptoms are often less marked or almost obscured as the fruit 
reaches maturity. 

The young fruits of mosaic muskmelon plants are often mottled, 
and a few dark-green warts may appear, but as the fruits mature the 
symptoms of the disease become less pronounced and the fruits are 
nearly normal in appearance. In the case of the Hubbard squash 
the young fruits of mosaic plants are sometimes mottled and lighter 
in color than those of healthy plants, but these symptoms are 
apparently obscured at maturity by the natural wartiness of the 
fruit. On the Summer Crookneck squash, however, the fruits are 



16 BULLETIN 870, U. S. DEPARTMENT OF AGRICULTURE. 

distinctly mottled, and the warty character is even more proiiouncei 
than on the (■ucunil)or. The symptoms differ from those on mos 
ciicurhits, liowevor, in that tlie raised portions of the fniit are lighte 
in color than the surroundin*; surface. Tlie contrast is often very 
prontmnced. the warts l)eing hright orange-yellow and the remaindei 
a dark green (PI. VI, B, C). 

Tlie fruits of the Large Cheese pumpkin (Cucurhita moscliatn) sho^ 
ver\' marked mosaic symj>toms, hut these symptoms have not as ye 
heen noted on any other variety of i)umpkin susceptihle to the disease 
Tlie young fruits are distinctly mottled ^vith light yellow, and tin 
surface is covered with large dark-green warts. (PI. V, C.) At matur 
ity the fruits are irregular in form, deeply furrowed, and the wart] 
chtu'acter is very pronoiniced. 

The fruits of most of the squashes, gourds, and the ornamental CU' 
curhits seem to he little affected hy the disease, and on many of thes< 
hosts no symptoms have as yet heen noted. The fruits of the mo 
saic wild cucumher { }f'i cramp elis JohaUi), however, are usually 
dwarfed, distorted in shape, and smoother than the fruits of normal 
plants, the spines l)eing fewer in numher and scattered unevenly ov 
the surface. In a few cases fruits have heen noted which devel 
oped rather large irregular swellings. The epidermis of the fruii 
eventually splits open at these points and the tissues helow push 
through to form dark-green sw^ollen areas, which are somewhat 
similar to those found on the cucumher. (See PI. VII, D and /''.) 
These symptoms apparently do not occur on all the mosaic fruits of 
this host, hut they have heen noted on a numher of plants. ITie 
fruits of the l-seeded hur cucumher (Sicyos nngnlatus) , on the other 
hand, do not seem to show any evidence of the disease. 

BLOSSOM SYMPTOMS. 

Mosaic cucumher flowers are not streaked or variegated as are those 
of tohacco plants affected with mosaic. Tliey are greatly reduced ini 
numher, however, especially the pistillate flowei-s. Blossoms pro-] 
duced in the later stages of the disease are dwarfed, the corolla often 
measuring not over three-fourths of an inch in diameter, and are 
slightly paler than normal hlossoms. On other cucurhits the symp- 
toms are of the same character, thoujrh the dwarfinj]: is usuallv leas 
pronounced except in the ciusc of the muskmelon, the hlossoms of 
which are usuallj' reduced in size and numher and are a much lighter 
yellow than those of normal plants. , 

I 

STEM SYMrrOMS. (I 

The stems in the case of the cucumher are shortened, as noted 
ahovc (PI. II, .4), and where the older leaves have died the ejiidermis 



^ 



Bui. 879 U. S. Dept. of Agriculture. 



Plate V. 




Mosaic Melon and Gourd Leaves and a Pumpkin. 

A, Mosaic leaf of a Lagenaria gourd, inoculated from cucumber; Madison, Wis., 1017. B, Mosaicleaf of 
a cantaloupe, natural infection; Big Kapids, Mich., 1U17. Photographed by W. W. Gilbert. C, Mosaic 
fruit of pumpkin, showing large dark-green swellings on a yellow background; Madison, Wis., 1919. 



Bui. 879. U. S. Dept. of Agriculture. 



Plate VI. 




LcAF AND Fruits of Mosaic Summer Crookneck Squash. 

'^k}^\l"i '/■'"'l'' •"'""""•■'■ V""'5"r'''' •'^'l'>n'='>. Mii'lison, Wis., 1917. B an.l C, VomiK fruits of mosaic 
.'*iimrn.T < rrM.kiHNk s-iunsli o^ ^^'^P^"'"^ "f »'"' ''iseiuso; B, from Irondequoit, 

N.Y. (photographed by W.W. (iillKTt),an<l r, from Xlii.lison Wi-; 1<»I7 ^ ' 



Bui. 879, U. S. Dept. of Agriculture. 



Plate VII. 




Mosaic Wild Cucumber Leaves and Fruits. 

"* Comnarf>\![h' r'' ^^r''%fJ(!"''""''''''l ( " ^"Z" '. -^IV""''"^ ™^'"'^'''^' mottling, malformation , and wrinkling. 
dSSiF Fn tsof l^'.r™ Y,"."?'?' ''■''^ "^ -U>cramiHlisiohafa. for comparison with' mosaic leaves. 
r^,■r:t,\uLu^ of l/;rrampf/;s/o6a?a from a mosaic plant, sliowiiig tlie irreRular shape and the wartliJ^e 

protuberances combined with the splitting of the outer integument; Madison, Wis. wartlike 



Bui. 879, U. S. Dept. of AErirulture. 



Plate VIM. 












Mosaic Cucumber Fruits. 

'*f.,«"Jmnnir.'.'"''n''.'!'T''i^ ^'''"r'' ""'"t';-l>''><l«'" .syn)i.toms, tlio color boitiR vollowish wliito with a 
nV^^ ^ ,.r ,M °' "ornnil Kmn ;< ut.,1 ( ■. Fruits witli Kn.-n wartlike i.rot iil.onince,s hirKt-r thaii 

o IS ,m I n s 11'"/ , / ,*"" . (•' !'''"-«.•;"« " fruit.s with .sevcrul lurge .l>irli-Kr..,.n warts a.i.iinimcr- 
w"k m. " • nt U JhlnBio, "'/"c ) "^ ^"^ "^ ^^ ^*^''-'" '*'**^''- ^^ ^^ ^' P''"t<H,TaphoU at Madison. 



THE MOSAIC DISEASE OF CUCURBITS. 17 

becomes whitish gray in appearance (PI. II, C). This leafless and 
whitened condition of the basal parts of the stem is one of the most 
easily recognized symptoms in the late stages of the disease on the 
cucumber. Such stems seem more brittle than those of normal 
plants and break more easilj^ Tlie whitening is due apparently to 
the loss of leaves and resultant exposure to the weather, since it 
occurs on healthy plants if the lower leaves are removed. The cu- 
cumber stem is not mottled, but it often develops a yellowish green 
color, somewhat lighter than the dark green of normal stems. Some 
cucurbits, particularly the squash and pumpldn, occasionally show a 
slightly mottled appearance of the stem in addition to the shortening 
of the internodes found on all cucurbits. This mottling consists of 
pale-green circular patches, about a centimeter in diameter, which 
occasionally have a slightly sunken appearance. 

ROOT SYMPTOMS. 

The roots of mosaic cucumber plants show no external evidence 
of the disease except in its later stages, when the number of finer 
rootlets is much reduced and the larger roots tend to show a faint 
yellow color accompanied by an unusual brittleness. Other cucur- 
bits examined have shown no differences between the roots of 
healthy and diseased plants except for a reduction in the number 
of finer rootlets. 

PATHOLOGICAL ANATOMY. 

ROOTS AND STEMS. 

Neither the roots nor stems of mosaic cucurbits, so far as examined, 
show any internal symptoms of the disease. There are no lesions, 
and sections of these parts have the same appearance as those of 
normal root and stem tissues. 



The leaves of mosaic cucurbits exhibit many variations in their 
external symptoms, the size of the yellow areas varying with different 
hosts, but the internal s3^mptoms are essentially the same in all 
cases. The dark-green portions are slightly thicker than the normal 
leaf tissue, and this fact accounts for the blistered and distorted 
appearance which is very marked in the leaves of cucumbers and 
squashes. The intervening yellow areas are somewhat thinner than 
the green portions, but are of nearly the same thickness as the tissue 
of normal leaves. 

When stained sections of the leaf are examined, the paUsade cells 

of the green areas are found to be crowded closely together and to be 

somewhat narrower and longer than those of the normal leaf. The 

palisade cells of the yellowed portion, on the other hand, are more 

185118°— 20 2 



18 BULLKTIN Xl\>, V. S. DKPARTMENT OF AGRICULTURE. 

iKMirlv isodiaincJric and loss in ininilxM- per nnit aron than in the 
normal leaf. 

The sj)on}j:y |)aroncliyma of these yellowed areas is also more 
compact and tlie intercellular spaces smaller than in the green 
portions. 

The chloroplasts of the cells of the yellowed portions arc decidedly 
smaller than in normal cells and are often ])ressed so closely to the 
walls of the cell as to be almost invisible. In contrast to this the 
chloroplasts in the dark-green areas are so large and numerous as to 
seem to fill the cells. The vascular elements of mosaic leaves show 
little variation from the normal. 



On the fruits, the dark-green warty growths and the abnormally 
light yellowish green of the surrounding surface are the chief external 
symptoms. Here, the cells directly beneath the epidermis assume 
the same abnormalities that occm' in the palisade cells of the dis- 
eased leaves. In the raised green areas, these cells are slightly I 
longer and narrower than in normal fruits, and the chloroplasts are 
larger and crowded closely together. In the 3'ellow portions of the 
diseased fruits these subej^idermal cells are more nearly square in 
longitudinal section and their chloroplasts are much reduced in size. 

The other tissues of the fruit, including the vascular elements, 
appear to be normal, the raised character of the green tissues being 
evidently a result of hyperplasia rather than hj'pertrophy. 

CAUSE OF THE DISEASE. 

The cause of cucurbit mosaic, like that of all other infectious 
chlorotic diseases, is unknown. WTiile the infectious character of 
the trouble is readily proved, there is no definite indication of the 
nature of the infective principle or its origin. 

RELATION TO SOIL AND WEATHER CONDITIONS. 

Extended observations on the mosaic of cucumbei-s in the lield 
have shown that the physical or chemical nature of the soil a])par- 
ently has no relation to the origin of the disease. Mosaic \vill de- 
velop with equal rapidity on clay, muck, or sandy soils, and a|)|)lica- 
tions of lime, wood ashes, stable manure, or commercial fortilizere 
have little appreciable effect. It has also been impossible to con- 
nect environmental conditions with the disease in any way which 
indicates that it origimites from any unfavorable condition of soil, 
temperature, or humidity.' 

' Recent InvestiRutlons iinlicate that envlronmontal conditions, particularly temperaturp, may affect 
the Buaceplibillty of the plant to mosaic, ami it is therefore probable that the rapidity with which the 
dUeose spreads is partially determined by the conditions of enviroumont. 



I 



THE MOSAIC DISEASE OF CUCURBITS. 19 

NO VISIBLE CAUSAL ORGANISM DISCOVERED. 

CULTURAL STUDIES. 

No parasitic organism has been found to be constantly associated 
with mosaic plants, although many cultures have been made from 
roots, stems, leaves, and fruits. In attemjjting to isolate a causal 
organism a great variety of media of all types, both vegetable and 
synthetic, were employed, particular use being made of those con- 
taining the juices of the cucumber plant itself. Tissue cultures 
from all parts of the plant remained sterile in most cases, and where 
grow^th occurred it resulted only from contaminations, no single 
organism being constantly present. Variations in temperature and 
oxygen supply and in acidity of the media were also tried, but these 
experiments were equally unsuccessful. 

The possibility of an increase of the virus in the culture medium 
without visible growth has been tested in many cases by inoculation 
from the medium itself, but no infection has ever appeared. The 
juices of mosaic plants may produce infection after passing through 
a Berkefeld filter, and attempts were therefore made to make cul- 
tures from these filtrates on various media, but without success. 
Similar filtrates of the juices of healthy plants when inoculated from 
infectious filtrates have never shown evidence that the virus could 
increase on such a medium. All attempts to cultivate the virus 
from the filtered juice of mosaic plants are also complicated by the 
fact that the juice loses its infectious character within two to three 
days after it is expressed from the tissues. 

ABSENCE OP ORGANISM IN DISEASED TISSUES. 

The absence of a visible causal organism is further supported by 
the fact that stained sections made from all parts of mosaic plants 
have given no indication of the presence of any parasite in the tissues. 
No differences have been detected between the stems and roots of 
healthy plants and those affected with the mosaic disease. The 
leaves and fruits show some morphologic differences, but no lesions 
of any sort are present and no visible organisms have been found in 
either the fruit or leaves of inosaic plants. All data so far collected, 
therefore, indicate that the disease is highly infectious, but no causal 
factor can as yet be associated with infection. On tlie other hand, 
the expressed juices of mosaic plants possess definite properties 
related to their power of infection. 

NATURE AND PROPERTIES OF THE MOSAIC VIRUS. 

The study of the nature of the infective principle of mosaic dis- 
eases has been most extensive in the case of tobacco, many workers 
having contributed to a knowledge of the properties and character 
of the virus concerned. While the writer's experiments wath the 



20 



BULLETIN 879, U. S. DEPARTMENT OF AdRHTLTURE. 



virus of tho mosaic of cucurbits havo boon Ic^s oxtonsivo, thoy indi- 
cMto tliat it is similar in many rospocts to tliat of tol)acco and other 
j)ljnits. Tho ])owcr of infection may bo destroyed in each case and 
is (h'linitely connected with tomperaturo, moisture, age, and other 
factors. 

EFFECT OF IIKAT oN TIIK I'OWKK (»K INFECTION. 

The expri>ssed juices of mosaic cucuml)er plants lose their powor 
of infection when lieated above 70° C. Tliis temperature has proved 
tho limit in all exporinients, as is shown in Table III. In thase tests 
tlie ]>lant jnictvs were expressed and filtered throuf^h filter paper 
b(»foro u.s«v Small test tubes were used and 5 c c. portions of tho 
juice taken to insure ra])id heating to the desired temperature. A 
constant-tempivaturo water bath was used and the tubes heated 10 
minutes and tlieii immediatt^ly cooled to room temperature. Inocu- 
lations were made, from the different tubes to healthy cucumber 
plants, inoculating into the stem, at the tip and base, and also into ■ 
two of the 3'oung leaves. These plants were protected from possible 
outside infection by covering them with insect-proof cages. 

Table III. — Effect of heat, on power of infection of expressed juices of mosaic cucumber 

plants. 





Temperature (°C.). 


Number 
of plants 
inocu- 
lated. 


Results. 


Date. 


Number 

of masaic 

plants. 


Date la.<!t 
observed. 


Field testes: 

Sept. 3 , 1916 


45 
50 
5.5 
60 
65 
70 
75 
80 

I'nheated. 
60 
65 
70 
75 
80 
85 
90 
95 

Unhealed. 

55 
60 
65 

70 
75 
SO 
85 
90 
Unhealed. 


3 
3 
3 
3 
3 
3 
3 
3 
3 

6 
6 
6 
6 
6 
6 
6 
6 
6 


2 
2 
3 

2 
2 


3 
3 
2 
4 





4 

4 

5 
2 
3 




4 


Sept. 18,1916 
Do. 


Do 


Do 


Do. 


Do 


Do. 


Do 


Do. 


Do 


Do. 


Do 


Do. 


Do 


Do. 


Do 


Do. 


Aug. 9, 1917 


Aup. 21,1917 
Do. 


Do 


Do 


Do 


Do 


Do. 


Do 


Do. 


Do 


Do 


Do 


Do. 


Do 


Do. 


Do 


Do. 


(irccnhou.se te.sts: 

Oct. 30. 1917 


Nov. 10 1917 


Do 


Do. 


Do 


Do. 


Do 


Do 


Do ::;■ 


Do. 


Do 


Do. 


Do 


Do 


Do 


Do. 


Do ;:.■.■ 


Do. 







I 



EFFECT OF AGE ON THE POWER OF INFECTION. 

The oxpro-ssed juic&s of mosaic cucumber and other cucurl)itaceou8 
phints retain the power of infection for only a short period iifter their 
(extraction. Ii(^p(nt(*d tests havo shown that tlie juices of mosaic 
plants of cucumbtM-, sfjuash, ])iimpkiii, muskmeion, and 'Micniiupdis 
Uibafa are never infectious for more tliaii tiiree to five days and in most 
ca.ses lose their virulence witliin 24 to 4S hours. This is in sharp 
contrast with results secured by Allard (4) with the juices of mosaic 



THE MOSAIC DISEASE OF CUCURBITS. • 21 

tobacco plnnts, whicli often remain infectious for more than a year, 
even without the addition of preservatives. In the case of cucurbit 
mosaic the juices undergo rapid fermentation unless a preservative 
is added, but various chemicals, such as chloroform, ether, toluene, 
and glycei'ino, used at different strengths have never served to 
increase the time during which the juices I'emained infectious. Low 
temperatures have only a sUght effect in prolonging the power of 
infection, and filtrates from Berkefeld filters in which no visible 
bacterial growth occurs are equally short lived in this regard. The 
preservatives used with the unfiJtered juices were also tried with 
Berkefeld filtrates, but neither chemical preservatives nor low 
temperatures have any noticeable effect in prolonging their power 
of infection. 

EFFECT OF DRYING ON THE VIRUS. 

As the juices of mosaic plants rapidly lose their power of infection 
with age, it Vv^ould be expected that drying would also remove this 
infective character. Tliis has been the case with the tissues of mo- 
saic cucumber plants. The leaves of mosaic plants when dried at 
room temperature for periods of 10 days to 1 year have always 
failed to produce the disease. In these experiments the dried leaf 
tissues were pulverized and allowed to stand from 8 to 24 hours in 
a small quantity of sterile distilled water. The water extract was 
then filtered off and inoculations made both by the injection of the 
water extract into the stem and leaves of healthy plants and also 
by the insertion of the ground and moistened leaf tissue in wounds in 
the stems of healthy cucumber plants. A total of 49 inoculations have 
been made by these methods, but no infection has ever occurred. 

In addition to inoculations with dried leaf tissue, the dried stems, 
small fruits, and roots of mosaic plants were used in the same wa}^ 
as inocula in 38 inoculations, but always with negative results. The 
expressed juices of mosaic plants also lose their power of infection 
on dr3dng. Experiments were made in wkich these juices were dried 
either on glass or on filter paper at room temperature. At intervals 
of seven days the dried material was taken up in sterile distilled 
water and the water extract pricked into the stems and leaves of 
healthy cucumber plants. Ten plants were inoculated from the 
dried material on glass and also from that on filter paper at the end 
of every seven days, the experiment being continued for five weeks, 
but no infection resulted from any of the inoculations. The result 
of experiments on soil transmission of the disease (p. 48) as well as 
the above data all indicate the rapid loss of the infective principle 
in the dried tissues of mosaic plants. 

* EFFECT OF DILUTION ON THE VIRUS. 

The power of the virus of cucurbit mosaic to increase rapidly 
after its injection into the host tissue has been indicated by the 
rapidity of its distrilmtion through the tissues of the plant inocu- 
lated. Further evidence of this rapid increase appears in the results 



22 



BITLI.KTIN 879, IT. S. DEPARTMENT OF AGRICULTURE. 



of iiiociilalions witli diluted juices of mosaic cucumljcr ])Iants 
Allan! (2) showed that the virus of tobacco mosaic could he dilute^ 
to 1 : 1,000 Nvilhout reduciii*; its virulence. He also made a numher 
of successful inoculations with the dilutions of 1:10,000, although 
higher dilutions rarely gave infection. The work of tlie writer with 
cu(um])er mosaic has given similar results, as is shown in Talile IV. 
The juices of mosaic plants were expressed, filtered through filter 
paper, diluted with sterile distilled water up to 1 :100,000, and used 
for inoculating healthy plants. These experiments show that dilu-i 
tions of 1:1,000 are as potent as undiluted solutions, hut while! 
infectious may result from those of 1 : 10,000 they have never taken 
place at higher dilutions. In all of tliis work inoculations were made 
at the hase of the stem and in two or thiee of the younger leaves, 
the latter heing inoculated at two or three points by pricking a drop 
of the solution into the leaf. Stems were cut slightly and the cut, 
surface was covered with a drop of the diluted virus. 

Tahi-k \\ .—Effect of dilution vith sterile distilled ivater on the power of infection of th 
expressed juices of mosaic cucumber plants. 





1 

Number 
Dilution of cxpressfJd juices. °[^'^.^® 
1 luted. 

i 


Results. 


Date inoculatod. 


Number 
oJ mo- 
saic 
plants. 


Disease 
noted 
(date). 


Series!: 

Apr. 8, 1916 


1 

Undiluted 5 

1:100 5 


3 
3 
3 
2 


3 
3 
2 



5 
6 
4 
2 



Apr. 20,1916 
Da 
Da 


Do 


Do 


1:1,000 5 


Do 


1:10,000 5 


Da 


Do 


1:100,000 5 


Series U: 

Sept. 8, 191(5 


Undiluted. 4 


Sept. 18,1916 
Da 
Da 


Do 


1:100 . 4 


Do 


1:1, (KX). .. 4 


Do 


1:10,000.... . 4 


Do 


1:100,000 4 




Serins III: 

May 15, 1917 


Undiluted S 

1:100 8 


May 23,1916 
Da 
Da 
Da 


Do 


Do 


1:1,000 1 8 


Do 


1:10,000 ' i 8 


Do 


1:100,000 1 ' s 












The regularity of infection which follows inoculation with dilutions 
of 1:1,000, together with the numerous infections at 1:10,000, shows 
an apparent power of rapid increase in tii(> infectious material present 
in mosaic plants, since in these ca.ses the period of incubation was no 
longer than when the undiluted juices were used. 

KFFEer OF VAHKdS CHKMICALS ON THK VIRUS. 

Experiments with the effect of various chemicals on the virus of 
cucurbit mosaic show that the power of infection is easily destroyed 
by c<^>mmon disinfectants and antiseptics. In this work the juices 
of mosaic cucumber plants were expressed and filtered through filter 
paper. The extract was put into test tubes in 5 c. c. portions and 



THE MOSAIC DISEASE OF CUCURBITS. 



23 



the desired chemical added in such amount and strength that when 
combined with 5 c. c. of the plant juices it gave the desired concen- 
tration. The tubes were then shaken well and allowed to stand for 
12 hours. Inoculations were made with these inocula, as in the case 
of the dilution experiments, into both the stems and leaves of healthy 
plants. The results sho\\T[i in Table V indicate that formaldehyde, 
phenol, mercuric chlorid, and copper sulphate in dilute concentra- 
tions were all toxic to the mosaic virus. 

The tests with copper sulphate gave definite mosaic infection in 
one case, but the writer is of the opinion that this is not an indica- 
tion that the juices are resistant to its action, since the same and 
weaker dilutions have destroyed the virus in all the other tests. The 
results in this single case, however, were apparently not due to out- 
side infection, as the j)lants were caged from the beginning of the 
season. Cliloroform in a 10 per cent mixture destroys the activity 
of the virus, but a 5 per cent cliloroform mixture seems to be 
harmless. Toluene in a 10 per cent mixture has no apparent effect. 
In the case of these chemicals special care was used to mix the solu- 
tion thoroughly, and the tubes were kept tightly corked until inocu- 
lations were made. The 10 per cent solution represents a large excess 
and the 5 per cent a slight excess beyond the possibilities of absorp- 
tion by the plant juices. The results of these tests have been 
consistent in most respects, however, as is shown in Table V, and 
indicate that the virus is not resistant to ordinary disinfectants. 

Table V. — -Effect of chemicals on the power of infection of the expressed juices of mosaic 

cucumber plants. 



Date inoculated. 



Chemical and strength used. 



Number 
of plants 
inocu- 
lated. 



Results. 



Number 

of mosaic 

plants. 



Date ob- 
served. 



Series I (field experiments): 

Aug. 10, 1917 

Do 

Do 

Do 

Do 

Do 

Do 

Do 

Series II (greenhouse ex- 
periments): 

Oct. 25, 1917 

Do..." 

Do 

Do 

Do 

Do 

Do 

Do 

Series III (greenliouso ex- 
periments): 

Mar. 28, 1918 

^ Do 

Do 

Do 

Do 

Do 

Do 

Do 



Formaldehyde 1 per cent.. 

Phenol 1 per cent 

CUSO4 1 per cent 

HgClo 1:1,000 

Chloroform 10 per cent 

Toluene 10 per cent 

Untreated juice 

Distilled H2O (control).... 

Formaldehyde 1 per cent.. 

Phenol 1 per cent 

CUSO4 1 per cent 

HgCls 1:1,000 

Chloroform 10 per cent 

Toluene 10 per cent 

Untreated juice 

Distilled H2O (control).... 

Formaldehyde 0.5 per cent 

Phenol 0.5 per cent 

CuSOj 0.5 per cent 

HgCl. 1:2,000 

Chloroform 5 per cent 

Toluene 5 per cent 

Untreated juice 

Distilled HjO (control) . . . . 



Aug. 20, 191- 
Do. 
Do. 
Do. 
Do. 
Do. 
Do. 
Do. 



Oct. 31,1917 
Do. 
Do. 
Do. 
Do. 
Do. 
Do. 
Do. 



Apr. 20, 1918. 
Do. 
Do. 
Do. 
Do. 
Do. 
Do. 
Do. 



24 



BULUCTIN 87!>, V. S. DEPARTMENT OF AGRICULTURE. 



( IIKMK'AI.S AS VIUUH IiISINKKCTANTS K»K TIIK HANDS. 

In addition to tiio work with clicmicnls in direct combination 
witli mosaic juices, further tests were made U> determine tlie value 
of various solutions as disinfectants for the hands after handling 
mosaic plants. In these tests the hands were smeared witli the juices 
of a mosaic cucunil)er plant and then rinsed in the solution to be 
tested. The healthy plants were then handled in such a way as 
to bring the hands in contact with slight wounds, the fruits present 
were picked, small shoots were pinched off, and other wounds made. 
Formaldehyde, copper sulphate, phenol, and mercuric chlorid were 
all used in weak solutions, as shown in Table VI, and in addition a 
strong soap solution and a mere rinsing of the hands in distilled 
water were tried. 

Taum-: \ I. — Value of rcrious chemicah as hand disin/crfauls o/lrr handling mosaic 

ciiruiiihcr plants. 



l)atc inoai- 



Chomicols and strength ^ used as disinfectants. 



Number 
of plants 
handled. 



Aup. 17,1916 Formaldehyde 1 per cent. 

Do Formaldch.vdo 2 i>er cent. 

1)0 Phenol 2 [lorcciit 

Do I'hciiol.l percent 

Do HgClj 1:1,000 

Do Soap and water , 

Do Dbtilled water 

Do I'ntrcatod mosaic juices.. 

Au^'. IS, 1917 i Formaldehyde 1 percent. 

Do I'heiiol 1 p«'rcent 

Do ('uSO< 1 percent , 

Do HuCl, 1:1,000 

Do Soap and water 

Do , Distilled water 

Do Untreated mosaic juices.: 



Results. 



Numlier 

of mosaic 

plants. 



Date ob- 
served. 



Aue. 26, 1916 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 

Do. 
Ann 2S, 1917 

Do. 

Do. 

Do. 

Do. 

Do. 

Da 



4 



As no infection occurred after any of these treatments, it is probable 
that the dilution effect is as important as that of chemical action. 
Hands covered with the juices of mosaic plants, however, gave a liigh 
j)ercentage of infection when no wash was used. 



EI-FECT OF FILTRATION' ON THE VIUUS. 



lirrl-tfthl Jiltfr. — The presence of n liitcrahh' \ iius as tlje causal 
factor in cuciiibit mosaic lias ah-cady been th'monstratcd. lioth 
Jjij^ger (17) and the writer (1 1) have shown that the juices of mosaic 
cucumber plants retain their power of infection after passing through 
a lierkefeld filter. 

The work on this phase of tlie problem lias been continued by the 
writer, using both the lierkefeld and tli(> Chainberland types of liltei"S. 

In the tests with the lierkefeld filler the juices of mosaic cucumbers 
were expressed and passed lliidiigh double lilter i)aper. .Vfter this 



THE MOSAIC DISEASE OF CUCURBITS. 



25 



filtration the juices were at once passed through a sterile normal 
Berkefeld bougie, the bougie, receiving flask, and all connections 
having been previously sterilized with steam for 45 minutes at 15 
pounds' pressure. 

Owing to the large amount of finely divided material which re- 
mains in suspension in the expressed juices, the filtration process is 
very slow and with the Berkefeld filter 12 to 14 hours are necessary 
to obtain 200 c. c. of filtrate from a bougie 1 by 6 inches. The 
filtrate was removed to sterile test tubes or small flasks by means of 
sterile pipettes and inoculation made from each lot to beef bouillon, 
Tubes which proved free from bacteria were used for inoculation, 
inoculations being made with the unfiltered juice at the same time. 
Tliese were made by pricking the filtrate into the young leaves at 
several points and into a wound at the base of the stem. The 
results given in Table VII show that the filtration of the infectious 
juices did not destroy their power of infection, most of the filtrate- 
inoculated plants developing mosaic symptoms as rapidly as those 
inoculated with the unfiltered juices. 



Table VII. 



-Effect of fiUrafion through a Berkefeld (normal) filter on the infectnity of 
the expressed juice of mosaic cucumber plants. 



Date inocu- 
lated. 



Aug. 31,1915 

Do 

Do 

Sept. 6,1916 

Do 

Do 

May 12,1917 

Do 

Do 

May 18,1917 

Do 

Aug. 30,1917 

Do 

Do 



Treatment. 



Filtered mosaic juice 

Unfiltered mosaic juice 

Unfiltered healthy juice (control) 

Filtered mosaic juice 

Unfiltered mosaic juice 

Unfiltered healthy juice (control) 

Filtered mosaic juice 

Unfiltered mosaic juice 

Unfiltered healthy juice (control) 

Filtered mosaic juice 

Unfiltered mosaic juice 

Filtered mosaic juice 

Unfiltered mosaic juice 

Unfiltered healthy juice (control) 





Results. 


Number 






of plants 






inocu- 
lated. 


Niunber 
ofmosaic 
plant.s. 


Date last 
observed. 


6 


4 


Sept. 15, 1915 


6 


5 


Do. 


6 





Do. 


8 


5 


Sept. 14,1916 


/ 


5 


Do. 


8 





Do. 


S 


3 


May 21, 1917 


S 


6 


Do. 


8 





Do. 


10 


5 


May 31, 1917 


10 


6 


Do. 


5 


2 


Sept. 12,1917 


5 


3 


Do. 


5 





Do. 



Chamherland filter. — In contrast to the results with the Berkefeld 
filter, the Chamberland type of porcelain bougie has given filtrates 
that were noninfectious. A considerable number of inoculations 
have been made, using the filtrates from different types of Chamber- 
land filters, but no positive results have been obtained. 

Tlie earlier work was done with filters of the "F" and "B" types 
about 1 by 8 inches in size, the ''B" type being supposedly the finer. 
Later a set of smaller bougies was obtained which gave a gradation 
in porosity. These were five-eighths by 6 inches and were graded as 
L2, L3, L5, L7, L9, and Lll. The L2 and L3 grades were supposed 
to be permeable to the more minute forms of bacteria, and the other 



26 BITLLETIN 87f>, IT. S. DEPARTMENT OF AGRICUT.TURE. 

types wore supposedly of suHicuMit density to prevent the pjtssjige of 
visible organisms. 

It Wfts hoped thiit a eompurison of (lie iiltrfttcs from these (hlferent 
grades of filters woidd ])erhaps indicate the size of the particla^ 
responsible for infection, but the results did not warrant any con- 
clusion alonj^ this line, as no infection has occurred in a total of more 
than foo plants inoculated with these filtrates. The filters of the 
Chamberland type, being of porcelain, are all denser than the Berke- 
feld, and filtration is therefore much slower. There is a possibility, 
therefore, that the infective principle may be held back by absorp- 
tion, as the suspended material in the plant juices rapidly clogs 
the filter and covers its surface with a gelatinous layer. 

Other filters. — Allard (4) reports that the juices of mosaic tobacco 
plants become noninfectious if passed through a Livingston at mom- 
eter. A fdter made of a layer of powdered talc from seven-eightlis 
to li inches thick also gave a noninfectious filtrate. Similar talc 
filters have been used in experiments with cucumber mosaic, the 
layer of talc varying from three-fourths of an inch to 1 inch in thick- 
nass. Three trials have been made with these filters, and a total of 
37 plants have hcon inoculated from the fdtrates. Tlie results have 
been negative in all cases, however, and it is evident that the filtrate 
is rendered noninfectious, as in the case of tobacco. 

Iwanowski (16), Koning (21), and Beijerinck (6) have stated that 
the juices of mosaic tobacco plants do not lose their infectious nature 
when passed through Berkefeld and Chamberland filters, though 
Iwanowski found that only the first portion of the Chamberland 
filtrate was infectious. It is thus e^'ident that the infective principle 
of both tobacco and cucumber mosaic is of such a nature that it can 
be removed by filters of the finer tj'pes. 

ENZYTIS IN RELATION TO THE DISEASE. 

Since many writers have advan^^^ed the theory that enzyms, par- 
ticularly oxidases, arc in some way connected with the mosaic of 
tobacco, a few attempts were made by the writer to isolate a pos- 
sible causal enzym from the Juices of mosaic cucumber ])lants. The 
fact that the juices of such plants usually lose their })ower of infection 
^v•itl^n 24 to 48 houi-s has proved an almost insurmountable obstacle 
hi such work,. and so little has been done that it is not fcjusible to 
draw any definite conclusions. Tests of the juices of healthy and 
di.seiLsed 1)1 ants wlu<h have been passed through filter paper have 
shown the presence of oxidases and peroxidases in both cases, the 
guaiacum reaction being used. Very little thllerence in the intensity 
was noted, but the juices of diseased plants seem to show a slightly 
stronger reaction for both enzyms. Similar tests of both healthy 
and mosaic ])lant juiccw after ])assago through a Berkefeld lilter have 
shown a weaker test for both oxidase and ])eroxidase in all <ase8, 



THE MOSAIC DISEASE OF CUCURBITS. 27 

tho intensity in licaltliy and diseased juices after filtration being ap- 
proximately equal. Attempts have been made to precipitate an 
enzym with 45, 70, and 80 per cent alcohol, using the juices of mosaic 
plants filtered through filter paper. The precipitates gave peroxi- 
dase reactions in all cases, but inoculations made with solutions of 
these precipitates were never successful. 

COMPARISON OF CUCURBIT MOSAIC WITH TOBACCO MOSAIC. 

The properties of the virus of cucumber mosaic are strikingly like 
those attributed to the virus of tobacco mosaic. As already indi- 
cated, the symptoms of the two diseases are very similar, as are also 
the changes produced in the anatomy of the respective hosts. Also 
in both hosts the points at which successful inoculations can be made 
correspond with one exception, i. e., in tobacco root inoculations 
produce infection. The vigor of growth and age of the plants are 
important factors in the infection of both hosts. Insects and the 
handling of mosaic and healthy plants in pruning and picldng are 
responsible for much of the field transmission of tobacco as well as 
cucumber mosaic. The first symptoms appear in the young leaves, 
both in tobacco and cucumbers, although the virus is found in all 
parts of the stem and leaves, regardless of the development of mosaic 
symptoms. In tobacco the virus is present in the flower parts, in- 
cluding the placental column and integument of the ovule, as is 
shown by Allard (3). It is also present in the mature seeds, accord- 
ing to the same writer. In the case of the cucumber, the virus is 
present in the flower parts and in the immature fruits, but has never 
been found in the mature seed. The vascular system may be con- 
cerned in the distribution of the virus in both cases, at least in the 
writer's opinion, since the evidence on which the theory is based is 
much the same for both hosts. 

The work on the properties and nature of the virus of tobacco 
mosaic has been much more extensive than that on cucumber, and 
furnishes a valuable basis for comparison. The thermal death point 
as worked out by Beijerinck (6), Woods (32), Iwanowski (16), and 
later by Allard (4) seems to lie between 80° and 100° C. This is 
somewhat higher than that of cucumber mosaic, which is rendered 
nonpathogenic if heated above 70° C. The expressed juices of 
mosaic tobacco plants will retain their power of infection for a year, 
or more, in some cases, whether preservatives are used or not, and 
the dried leaves will remain infectious for relatively long periods. 
This is different from the disease on cucumbers, where the expressed 
juices are seldom infectious for more than 48 hom's and the dried 
tissues have never proved infectious. 

Disinfectants, such as phenol, formaldehyde, mercmic chlorid, and 
copper sulphate, mil destroy the virus in both cases, tho virus of 
tobacco mosaic apparently being shghtly more resistant in this 



28 BULLICTIN 87i>, U. S. DEPARTMENT OF AGRICULTURE. 

regard. 'IMic juices of both luosiiic tobacco and cucumber ])lant8 
may be diluted to 1 : 1,000 \\'ith(»ut aHectin*^ the power of infection, 
and dilutions oi 1 : 10,000 will also ])roiluce the disease, ^ivinj?, how- 
ever. H lower j^ercenta^e of infected j)lants. 

Tln' tobacco \'irus will pass through both the normal Berkefel 
filters and those of the Chamberlantl type, while the cucunil^er virui 
passes only tlu'ough the former. This diderence is probably of mino: 
importance, however, as Allard (4) has shown that the juices ol 
tobac<-o mc^saic are also rendered noninfectious if passed through i 
j)orous clay atnionieter or through u :{-inch layer of powdered talc 
so that the beliavior of the Airus of both diseases is essentially tht 
same as regards filtration. 

POSSIBLE NATURE Or THE CAUSAL FACTOR. 

Since no visible causal organism has been associated with the* 
mosaic diseases, various theories have been advanced as to their 
nature and origin. 

Woods (."^2), Koning (21), and more recently Freiberg (\'S) and 
Chapman (7) have held that enzjTns, particularly oxidases, peroxi- 
dases, and catalase, are in some way connected with the cause of 
tobacco mosaic. iUlard (4), on the other hand, has claimed that the 
disease is due to a specific pathogenic agent, probably an ultra- 
microscopic organism. 

Both theories are based principally on work with the mosaic of 
tobacco, but they apply equally well to the corresponding disease 
of cucumbers. The evidence so far accumulated, however, seems t 
accord better with the theory advanced by ^\llard than with th 
enzymic h>T>othesis. 

The virus of cucumber mosaic, like that of tobacco, seems to pos 
sess many of the characteristics of living matter. It loses its power 
of infection if heated above 70° C, is easily destroyed by chemicals, 
and will not withstand desiccation. In supj)ort of the enzymic 
theory of the nature of tobacco mosaic it is claimed that similar 
properties are possessed by enzyms, and this is undoubtedly true to 
a great extent. Tlie virus of cucumber mosaic, however, does not 
have such marked enzymic qualities. Unlike that of tobacco, it 
loses the power of infection within 24 to 48 hours after the juices are 
expressed from th(» plant, regai-dless of the use of preservatives or the 
temperature at which it is kept, and will not withstand desiccation. 
It is quite conceivable, however, that an organism might be destroyed 
rapidly after removal from its natural environment in the plant tis- 
sues, especially as the juices of the cucumber undergo rapid chemical 
changes when expressed. 

Another point whicli seems to support tlie tlieory of an organism 
as the cause of mosaic is th<' ability of tlie juices of mosaic plants to 
prudiicr infection in a dilution of 1 : 10,000. IVo or three drops 



THE MOSAIC DISEASE OF CUCURBITS. 29 

of such a dilution when pricked into the leaf of a health}^ plant wiJl 
produce the disease as rapidly and with as intense symptoms as 
when the undiluted juices are used. We have evidence here that the 
virus possesses the power of rapid increase, since all parts of the 
leaves and stem of the inoculated plant contain the virus within thi'ee 
to four days after inoculation. If we adopt the theory of an ultrami- 
croscopic organism as the causal factor, this increase is readily ex- 
plained, but examples of similar increase or multiplication are not 
commonly associated with enzyras. 

The behavior of the virus in filtration may be used to support the 
theory, although the fact that the fmer filters hold back the virus 
indicates that the particles composing the virus are probably colloidal 
in nature and of relatively large size. These qualities, however, 
neither exclude it from the class of enzyms nor from that of ultra- 
microscopic organisms. 

In attempting to prove either of the above hypotheses it must be 
recognized that a great part of the available data is equally applicable 
to either theory and that no definite conclusion is as yet possible. 
It is the writer's opinion, however, that the causal agent possesses 
characteristics which tend to place it as an ultramicroscopic organ- 
ism rather than as an enzym, but no property has yet been discovered 
which may not be characteristic of either agency. 

On the other hand, the examples of diseases attributed to a so- 
called "filterable virus" or ''ultramicroscopic organism" are numer- 
ous in animal pathology and are generally accepted, while as yet 
there are no demonstrated examples of diseases due to enzymic 
causes alone. In the case of tobacco mosaic the recent work of Allard 
(4) has nearly eliminated the oxidases from consideration, so that we 
at present can hardly attribute the disease to a definite type of 

enzym. 

INFECTIOUS NATURE OF THE DISEASE. 

Tlie epiphytotic character of cucumber mosaic and its rapid spread 
to plants adjoining those first infected at once indicated that it was 
of an infectious nature, as proved by Jagger (17) and the writer (11). 
The first inoculation experiments by the writer were begun at Ham- 
ilton, Mich., in 1914, but as the plants used had no protection from 
insects and mosaic was already present in the field the results, although 
positive, did not permit definite conclusions. In 1915 the work was 
continued at Big Rapids, Mich. This district was practically free 
from the disease and the experimental fields had never before grown 
cucumbers. The possibility of outside infection, particularly from 
insects, was practically ehminated by using cages covered with cheese- 
cloth to protect all plants in the inoculation experiments. 

As the cages in most cases had no openings, they were lifted when 
inoculating the plants, but by exercising proper care and using the 



30 BUIJ.KTIN H7!), V. S. nF.PARTMKNT OF AGRICULTURE. 



I 

•aircS 



cage as a partial shield (lurin<j; inoculation insects gained entran 
very rarely. It was necessary to insjx'ct all cages frerjuently to cut 
the tendrils which penetrated tlie cloth and to cover any small open| 
ings witli lieavy paint. All cages were set out as early in the seasorf 
as possible, usually immediately after planting or by tlie time two 
to three leaves had appeared on the young plants.' Wlien plant* 
were caged after the disease appeared in tlie field tliey were left af 
least 10 days before being used in inoculation experiments, in order 
that any previous accidental mosaic infection might have time to 
develop before inoculations were made. 

Most of the cages used were of the same general type, the frame 
being of laths, with the exception of the comer uprights which we 
made of pieces from 1 to 2 inches square. In the earlier cages ext 
strips of lath were placed around the lower edge of the frame to allow 
it to be sunk 2 to 3 inches in the soil, but it was later found advisabl^ 
to make the lower edges of liglit 4-inch boards. The cheesecloth 
used contained 24 to 30 threads per inch, the lower edge of the cloth 
being WTapped around strips of lath, which were then nailed to th^ 
bottom of the frame. Cages were also constructed with an opening 
in the top, but as they were difficult to build and often failed to remam 
insect proof, they were later abandoned, except for special purpos 
Cages were used in practically all field work and reduced the fact 
of outside infection, whicli occurred only in rare cases, to a minimum! 
The caged plants grew luxuriantly and were only slightly affected by 
the shadinsr of the cheesecloth. 



f 



3] 

lYll 



INOCULATION EXPERIMENTS. 

Inoculation of healthy vines with the juices of mosaic plants has 
proved that the causal agent of the disease is present in the tissues 
of the stems, leaves, and fruits of mosaic vines. A light woundinj: 
of the plant inoculated is appai'cntly essential for infection, as the 
virus does not seem to penetrate the unbroken epidermis. The two 
methods of inoculation used differ cliiefly in the form of the inocu- 
lum, the expressed juice of mosaic plants or their crushed tissues 
being used. Results by either method have been successful, as ie 
shown below. Control inocuhitions were made in all cases, using 
caged plants and inoculating them with the expressed juices or 
crushed tissues of healthy plants (PI. IX, A). 

TESTS WITH EXrnKSSED JUICES OF MOSAIC PLANTS. 

In the earlier work the expressed juices of mosaic plants were used 
for inoculation. Portions of a mosaic plant, cither leaves, stems, or 
fruits, were cut finely by means of a sterilized meat chopper, the 
juice pressed throuj^h clieesecloth, and in some cases filtered through 
liltcr ])aper, and the expressed juice used as the inoculum. The 



THE MOSAIC DISEASE OF CUCURBITS. 



31 



inoculations were made in the stems, leaves, and in some cases in 
the fruits. In the earlier experiments a sterile hypodermic needle 
was used for inoculation and the inoculum injected rather deeply 
into the stem. It was found, however, that inoculations made by 
cutting off a leaf close to the stem and then pricking the inoculum 
into the wounded surface gave a higher percentage of infection. In 
leaf inoculations a drop of the inoculum was pricked into the leaf at 
various points with a sterile needle. The results of inoculations by 
these methods are shown in Table VIII. 

Table VTII. — Results ofinocidations xoith the expressed juices of mosaic cucumber plants. 



Date 
inoculated. 



Inoculum, 

expressed juice 

of— 



Point of 
inoculation. 



Num- 
ber of 
plants 
inocu- 
lated. 



Results. 



Num- 
ber of 
mosaic 
plants. 



Date last 
observed 



Au-. 2, 1915 

Do 

Aug. 4, 1915 

Do 

Aug. 21,1915 

Do 

Nov. 19,1915 

Do 

Nov. 28, 1915 
Apr 8, 1910 

Do 

Sept. 6,19i(i 

Do 



Inoculated . . 

Control 

Inoculated . . 



Control 

Inoculated... 

Control 

Inoculated... 

Control 

Inoculated . . . 

do 

Control 

Inoculated. .. 
Control 



Mosaic plant . . 
Healthy plant. 
Mosaic fruit . . . 



Healthy fruit . . 

Mosaic "fruit 

Healthy fruit . . 
Mosaic leaves.. 
Healthy leaves. 
Mosaic plant... 

do 

Healthy plant . 
Mosaic plant . . . 
Healthy plant.. 



Base and tip of stem. 

do 

Young leaf and base 
of stem. 

do 

Base of stem 

do 

Young leaf 

do 

.do. 



Base of stem. 

do 

do 

do 



Aug. 17,1915 
Aug. 20,1915 
Aug. 14,1915 

Do. 

Sept. 2,1915 

Do. 
Nov. 26,1915 

Do. 
Dec. 6,1910 
Apr. 20,1910 

Do. 
Sept. 15,19ig 

Do. 



TESTS WITH CRUSHED TISSUES OP MOSAIC PLANTS. 

In later work, experience proved that the easily prepared crushed 
tissues of mosaic plants formed a more constantly virulent inocu- 
lum than the expressed juices. Infected portions of the plant, 
either fruit, stem, or leaves, were crushed in a sterile dish after they 
had been cut into small fragments with sterile scissors or a scalpel. 
This material was inserted in a small cut in the stem made with a ster- 
ile scalpel. Such wounds were usually made in the lower portion 
of the stem, in some cases directly below the oldest leaf, but more 
commonly a leaf was cut off close to the stem and the incision made 
i in the cut siu^f ace. A longitudinal cut, 3 to 5 millimeters long and 
I 2 to 3 millimeters deep, was found sufficient, the small piece of crushed 
^material being inserted in the wound, which healed rapidly. In 
I other cases a small incision was made at the tip of the stem directly 
' below a young leaf and a fragment of the crushed tissue inserted at 
this point. Inoculations with crushed material at either of these 
points, or both, ordinarily gave a slightly greater percentage of infec- 
]tion than was obtained by the injection of the expressed juices. It 
was found also that inoculations at two or three points were more 
generally successful than where a single inoculation was made. 



32 



BUU.KTIN H7!>, U. S. DKrARTMliNT OF ACKICULTURK. 



This method is proforrod chiefly hocause the preparation of the 
expressed juice re(iiiires time, and such extracts, unlike those from 
mosaic tobacco plants, lose their pathogenicity rapidly, usually 
haviu}^ to be prepared fresh every day to insure success. In the case 
of the second method, fresh material is constantly availa])le and its 
preparation takes little time. 

Table IX f2;ives results which are ty])ieul of inoculations by this 
method, but represent only a small fraction of the inoculations actu- 
ally niiid(\ 
Table IX. — Results of inoculations with the crushed tismes of mosaic cucuviber plants. 



Drtfe 
inoculated. 



Trentmcnt. 



Inoculum. 



Point of inoculation. 



Number 

of 
plants 
inocu- 
lated. 



Rosolts. 



Number 

of 
masaic 
plants. 



Pate lust 
observed. 



.\ug. 24, 1915. 

Do 

Jan. 22,1916 
•N'ov. 3,1916 

Do 

Feb. 17,1917 

Do 

.Mar. 11,1917 

Do 

Mar. 15,1917 

Do 

Do 

Mar. 29,1917 

. Do 

May 19,1917 

Do 

Jime 20,1917 

Do 



Inoculated. 

Control 

Inoculated. 
do 



Masaic .stem... 
Healthy stem. 

Mosaic leaf 

do 



Base of stem . 

....do 

do 

do 



Control 

Inoculated. 

("onlrol 

Inoculated. 

Control 

Inoculated, 
.do. 



Control 

Inoculated . 

Control 

Inoculated . 

Control 

Inoculated. 
Control 



Health V leaf.. 
Mosaic leaf... 
Healthy leaf.. 
Mosaicleaf... 
Healthy leaf.. 
Mosaic stem.. 
Mosiio fruit .. 
lloallhy fruit. 
Mosaicleaf... 
Ilcalliiy leaf.. 
Mosaic leaf. .. 
Healthy leaf.. 
Mosaic leaf. . . 
Healthy leaf.. 



do 

Tip of stem. 
,do. 



I$a.se and tip of stem.! 

do I 

Base of stem 

do 

do 

Base and tip of stem 

do 

do 

do 

Tip of stem 

do 



Sept. 2,1915 

Do. 
.Tan. 31,191f. 
Nov. 10, 191t) 

Do. 
Feb. 2S, 1917 

Do. 
Mar. 19,1917 

Do. 
Mar. 22,1917 

Do. 

Do. 
Apr. 9, 1917 

Do. 
May 30,1917 

Do. 
Julv 4, 1917 

Do. 



RELATION OF INFECTION TO POINT OF INOCULATION. 

STEM INOCULATIONS, 

As shown by the preceding data, stem inoculations are successful 
in most cases, regardless of the point of inoculation. Inoculations 
made in wounds produced by the removal of a leaf are ordinarily 
more effectivo than at other points, possibly because the virus comes 
more directly in contact \vith the vascular bundles at such points. 

LEAP INOCULATIONS. 

Inoculation of any green leaf will produce mosaic infection, but the 
inoculation of young leaves gives a much higher percent.age of mosaic 
plants than the inoculation of loaves that have reached their full 
development. Th(^ point of inoculation in the leaf, however, seems 
to make little difference. 

An interesting fact, already proved by Allard (5) for tobacco mosaic, 
is that infection can take place through (he trichomes. He hivs shown 
that infection iiiiiy occur if the trichoiiu-.s of healthy plants aii' cut or 
pinched with instruments which have previously been dipptMl in the 
juices of a mosaic phint or when tlui mosaic juices jire pjiinted on the 
trichomes. Similar experiments have been made with the cucum- 



Bui. 879, U. S. Dept. of Agriculture. 



PLATE IX. 




■■--«■*. 



■-^ 



fb 



'^'■%t 










^^ 











Cucumber Experimental Field and Soil Overwintering Test. 

A, Experimental field at Big Rapids, Mich., in 1916, showing the various types of insect-proof cages used 
in niaking inoculations Photographed l^y W. W. Gilbert. B, Soil overwintering test at Madison, 
Wis S^tember 19 1917. This land grew a mosaic crop in 1916. The few large healthy plants were 
grown imde?cages' All the uncaged plants became diseased early and were dead or severely stunted. 
An insect hibernation cage is seen at the left. 



Bui. 879. U. S. Dept. of Agriculture. 



Plate X. 




Mosaic and Healthy Cucumber Plants from Commercial Seed. 

A, Mosaic plant ^rown f rom com iiiiTcial seed in aKreoiiluniscat Mailison. Wis.,in 1917. Note the dwarfing 
tt.scointmrt<l witli H, tlic inottlccl and curled leu vc.-;, tlic. shriveled cotyledons, and tlie<lyinp oft lie edges 
of the first leaf, li, A healthy plant grown from the .same seed as A "and planted on the same date. 



THE MOSAIC DISEASE OF CUCURBITS. 



33 



ber, the trichomes of mosaic and healthy leaves being brought in 
contact in such a way as to merely break th(^ trichomes without 
injuring the epidermis of the leaf. Infection has resulted in most 
of the plants thus inoculated, as is shown in Table X. 

Table X. — Infection as a result of contact of trichomes of leaves of healthy and mosaic 

cucumber plants. 





Treatment. 


Number 


Results. 


Pate 
inoculated. 


of 
plants 
inocu- 
lated. 


Niunber 

of 
mosaic 
plants. 


Date last 
observed. 


Apr. 20,1916 
Do 


Mosaic and ln'althy leaves in contact 


2 
2 
6 
4 
12 
12 
10 
8 
4 
4 
12 
6 


2 


4 

9 

.5 

2 

3 



Apr. 28,1916 
Do 


Healthy leaves in contact (control) 


Aug. 6, 1916 

Do 

Feb. 21, 1917 


Mosaic and healthy leaves in contact 


Aug. 10,1916 
Do. 


Healthy leaves in contact (control) 

Mosaic and healthy leaves in contact 


Do 


Healthy leaves in contact (control) 


bo ' 


Mar. 22, 1917 


Mosaic and healthy leaves in contact 


Apr. 4, 1917 
Do 


Do 


Healthy leaves in contact (control) 


Juno 20,1917 


Mosaic and healthy leaves m contact 


Jime 28 1917 


Do 


Healthy leaves in contact (control) 


bo ' 


Oct. 24,1917 
Do 


Mosaic and healthy leaves in contact 

Healthy leaves in contact (control) 


Nov. 2,1917 
Do 









BLOSSOM INOCULATIONS. 



No evidence has been obtained thus far wliich indicates infection 
through the blossoms. Numerous inoculations of male and female 
flower parts were made by pricking the expressed juice of a mosaic 
cucumber plant into the petals, stigmas, or anthers of the blossoms 
of healthy cucumber plants. From 3 to 8 blossoms on each of 38 
plants have been inoculated in this way, but no infection has ever 
occurred. It is still a question, however, whether such infection may 
not occur, especially as the virus is known to be present in the blos- 
soms of mosaic plants, as is shown on page 35. 



FRUIT INOCULATIONS. 



Inoculations into small fruits have given infection in a number of 
plants, as is shown in Table XI, but inoculations of fruits approaching 
maturity were nearly always unsuccessful. 

Table XI. — Results of inoculation of fruits of healthy cucumber plants. 





Treatment. 


Inoculum. 


Approxi- 
mate age 
of fruits 
inoculated. 


Number 
of fruits 

inocu- 
lated per 

plant. 


Number 
of plants 
inocu- 
lated. 


Results. 


Date inocu- 
lated. 


Number 

of mosaic 

plants. 


Date last ob- 
served. 


Mar. 20,1916 
Do 

Aug. 20,1917 
Do 


Inoculated . . . 

Control 

Inoculated . . . 
do 


Expressed juice of 
mosaic plant. 

Expressed juice of 
healthy plant. 

Expressed juico of 
mosaic plant, 
.do 


10 days.... 

..-.do 

5 days 

21 days.... 
5 weeks . . . 
do 


2 

2 

2 

2 
2 
1 
2 

2 

1 
2 


6 

S 

8 
3 
7 
10 

5 

4 

8 


3 

3 

3 



1 


3 

1 



Apr. 5, 1916 

Do. 
Sept. 5,1917 

Do. 


Do 


do 


do 


Do. 


Do 


.. ..do.. .. 


do 


Do. 


Do 

Apr. 12,1918 
Do 


Control 

Inoculated . . . 
.. .do 


Expressed juice of 
healthy plant. 

Expressed juice of 
mosaic plant. 

. .do 


10 days.... 

7 days 

21 da vs.... 
7 day's 


Do. 

May 3, 191S 

Do. 


Do 


Control 


Expressed juice of 
healthy plant. 


Do. 



;85118= 



34 BULLETIN 879, U. S. DEPARTMPIXT OF AGRICULTURE. 



HOOT INOCULATIONS. 



Attempts to j)r()(lii((' inosnic l)y inoculations through tho root 
have so far jjivcii iicfjativc results. Seventy-four plants luwo booi 
inocuhitcd, using the following threes methods: 

(1) Inoculation of the roots was made ^vith the exprcsned juice of a mosaic plan 
or by tho insertion of crunhod fra<,'nicnt.s of mosaic leaves in wounds in the larger root 
at a distance of 1 to G inches from the stem. 

(2) Mosaic vines m ere passed Ihroui^h a food chopper and the ip'ound material mixed 
with sterilized soil. Healthy plants were then transplanted in the soil thus prepared, 
the process of transplanting insurin;? root injuries as points of possible infection. 

(3) Roots of healthy plants were injured at points close to the surface of the soil, 
and the expressed juice of mosaic plants was then poured about the stem cloee to the 
point of injur)^ 

No infeetion has resulted from any of these methods, and the dis- 
ease apparently is not transmitted through the roots. 

RELATION OF AGE AND VIGOR OF PLANT GROWTH TO SUSCEPTIBILFTY. 

The percentage of successful inoculations on the cucumber and 
other curcurbits seems to bo directly related to the age of the plant 
and tho vigor of its growth. Inoculations on seedling plants, even 
up to the time that three or four leaves have appeared, give a lo^ 
percentage of infection, and it is especially difTicult to secure infec- 
tion on seedlings during the development of the first true leaf. 

A little later, however, when tho plants begin to grow rapidly and 
have developed six to eight leaves, most of those inoculated develop 
the disease. This period of susceptibility then continues until the 
plant becomes old and growth has nearly ceased, at which stage the 
percentage of successful inoculations again becomes low. Plants 
trrowinj; under unfavorable conditions which cause a stinitiufr or 
retardation of growth are also less susceptible to the disease and show 
the fii*st symptoms more slowly than those which are growing rapidly. 

These facts indicate that the ago and vigor of the plant have a 
direct relation to its susceptibility to mosaic. Wliile this is probably 
true, it is a question what relation these factors boar to the suscep- 
tibility of the plant and its response to tho disease stimulus. The 
rapidly dividing cells of the tissues still in the process of developmen i 
apparently react to the disease stimulus more readily than tho older 
tissues. It is possible that these yomiger tissues offer conditions 
more favorable to tho multiplication of tho virus ' and consequent 
infection than tho older cells of mature |)lants. 

It has been sugg(>sted that old(>r ])lants may contain the virus but 
show the symptoms in so mild a form that they can not be detected. 
However, this has aj)parently been disproved by making inocu- 
lations from old plants which had been previously inoculated with 

I Tho term virus Is used here In tho commonly aooopt«d sonso of the lofooUvo agent o(a diaeaso with whicl< 
iiu visible organism hus boon iissocIuUhI, 



p 



THE MOSAIC DISEASE OF CUCURBITS. 



35 



mosaic but showed no symptoms of the disease. No such plant has 
ever shown evidence of containing the virus, as all inoculations from 
them gave negative results, and it is probable that a definite differ- 
ence in susceptibility occurs, depending on the age of the plant. 

LOCATION OF THE VIRUS IN THE PLANT. 

The virus of curcubit mosaic is distributed throughout practically 
all parts of the plant, with the possible exception of the roots. The 
data given in Tables VIII to X show that the juices of stem, leaves, 
and fruits of mosaic cucumber plants are all capable of producing 
infection. All the living leaves of a mosaic plant contain the virus, 
regardless of age or the presence of actual mosaic symptoms, its 
thorough distribution in the leaf cells being shown by the results of 
inoculation by contact of healthy and mosaic trichomes. 

VIRUS IN THE FLOWER PARTS. 

The disease can also be produced by inoculation with the flower 
parts of diseased plants. Inoculations were made with petals, 
anthers, and stigmas of mosaic cucumber plants by carefully dis- 
secting out with sterile instniments the portion of the flower desired 
without breaking other parts and crushing the niaterial in a sterile 
dish with a few drops of sterile water. This crushed mass was then 
used for stem inoculations, the results being positive in each case, 
as shown in Table XII. Allard (3) has shown that the virus of 
tobacco mosaic is also present in the anthers, filaments, and even in 
the placental column and integument of the ovule of infected plants. 

Table XII. — Results of inoculations with the crushed tissues of flower parts of mosaic 

cucumber plants. 





Treatment. 


Inoculum. 


Number 
of plants 
inocu- 
lated. 


Results. 


Date 
inoculated. 


Number 

of 
mosaic 
plants. 


Date 
observed. 


Sept. 7,1916 
Do 


Inoculated .' 

.. . do . . 


Corolla of mosaic blossom 


3 
3 
3 

10 
8 
8 
8 

10 
8 


3 
3 
2 
3 

5 

6 



Sept. 23,1916 


Stigma of mosaic blossom 


Do. 


Do.. . . 


do 




Do. 


Mar 12 1917 


do 


do 


Mar. 21,1917 


Do . 


Control 


Anthers of healthv blossom 


Do. 


Do 


Inoculated 




Do. 


Do 


StlRtna of healthy blossom 


Do. 


Do 


Inoculated 




Do. 


Do... 


Corolla of healthy blossom 


Do. 











VIRUS IN THE FRUITS. 



Inoculations made with the crushed tissues of mosaic cucumber 
fruits have shown that the virus is apparently present in fruits of 
all ages. The inoculum was prepared by removing small i)ortions 
of the tissues close to the ovule with a sterile scalpel and crushing 
them in a sterile dish, the inoculations being made in the stems of 



36 



BULLKTIN S7!», U. S. DEPART.MKNT OP' AGRICULTURE. 



licalthy cucuiiihrr plants by ihc liu-thod clt'scribcd on panics 30 lo 32, 
Tho fruitij used as inocula were of various a^es, mcludinj; young fruita 
about ]\ inclios in length, large, partially niatui-e fruits that were 
beginning to turn yellow, and yellow and matured fruits such as art 
collected for seed. The very young and the partially mature fruitj 
showed the presence of the vims in all cases (Table XIII), and whiU 
the inoculations from the mature fruits were not so constantly suc- 
cessful they produced the disease in a number of plants. The pres- 
ence of the virus in the ovule itself has not been demonstrated, as if 
is diilicult to remove the ovules from a watery fleshy fruit like the 
cucumber without carrying traces of tho juices of other parts of the 
fruit. 

Tahi.k XIII. — licsttlls n/ itioi-u lotions with crushed tissues of mosaic rucvmhcr fruits of 

various ages. 



r>atc inocu- 
lated. 



Treatment . 



Inoculum, cmshctl 
tissues of— 



Number 
Point of inocula- of plants 
tion. inocu- 

lated. 



Results. 



NumbCTJ Pate last 



Sept. 10, 1917 

Do 

Do 



Inoculated . 
do 



-do. 



Do 

Sept. 20, 1917 
Do 



Control 

Inoculated. 
do 



Do. 
Do. 



do.. 

Control . 



Young mosaic fruit 

Partly mature mosaic 

fruit. 
Mature, yellow mosaic 

fruit. 

Ilealthy fruit 

Vouni; mosaic fruit 

Partly mature mosaic 

fruit. 
Mature, yellow mosaic 

fruit. 
Healthy fruit 



Base and tip of 

stem. 
do 



-do. 



..do. 
..do. 
..do. 



-do. 
.do. 



Sept. 21, 1917 

Do. 

Do. 

Do. 

Oct. 14, 1917 

Do. 

Do. 

Do. 



VIRU.'^ IM THE M.\TUKK SEKI). 



Although the young fruits and even fruits at maturity may have 
the virus present, as is shown in Table XIII, it has never been de- 
tected in any portions of the seed which have been removed from the 
fruit and subsequently dried. The drying process probably accounts 
for its absence from the seed coat, since the juice of mosaic cucumber 
plants soon loses its infectious quality if subjected to desiccation. 
Tests have been made of both seed coat and embr^'o in 37 inocula- 
tions, the material being crushed in sterih* distilled water and both the 
water extract and the crushed tissue used for inoculation, but no infec- 
tion has ever occurred. Allard (3) has found that the virus of tobacco 
mosaic may bo present in the mature dry seeds of mosaic tobacco 
plants, and this is consistent with the fact that the virus of tobacco 
mosaic is more resistant to drying than that of cucurbit mosaic. At 
the present time, however, there is ratherdefinite proof that the mosaic 
disease of cucumlxM- may be carried over in the seed. (PI. I, A.) If 
such is the case, it is possible that the virus is occasionally present in 
the seed, altliough perliaps so rarely that a large number of inocula- 
tions would be necessary to demonstrate its j)resence. 



THE MOSAIC DISEASE OF CUCURBITS. 37 



VIRUS IN THE ROOTS. 



Inoculations from the roots of mosaic plants have so far given 
negative results, and it is doubtful whether the virus is present in 
these parts as in the remainder of the plant. Allard (1) notes that 
the roots of mosaic tobacco plants contain the virus in many cases. 
In the case of cucumber mosaic, inoculations have been made from 
the roots of plants of various ages, using either the expressed juice of 
the root tissues or crushed fragments of the tissues themselves. 
Inoculations have been made into the stems, leaves, or roots of 58 
plants, but no infection has occurred. 



VIRUS DISTRIBUTION IN THE PLANT. 

RAPIDITY OF SPREAD OF THE VIRUS. 



In all inoculations with cucurbit mosaic the earliest signs of infec- 
tion appear in the youngest leaves. This is also the case with to- 
bacco mosaic and other diseases of this type, the leaves which are in 
process of development seeming to respond most rapidly to the 
disease stimulus. The time required for the development of these 
symptoms is usually the same, regardless of the point of inoculation, 
indicating that the virus is rapidly distributed to all parts of the 
plant. The youngest leaves of the cucumber develop mosaic sjinp- 
toms as rapidly when inoculation is made at the base of the stem as 
when made in the leaf itself. This does not prove that the virus is im- 
mediately distributed throughout the plant, however, for the infective 
principle is present in the cells for some time before visible S3anptoms 
occur and it is probable that the virus is present in greater amount 
near the point of inoculation for at least a short time after inocula- 
tion. The older leaves, however, do not show definite signs of the 
disease for some time, and no external indication of the disease ap- 
pears until the young leaves develop mosaic symptoms. The accu- 
mulation of the virus at the point of inoculation earlier than at the 
growing tip has been demonstrated in large cucumber plants where 
inoculation was made at the base of the stem about 3 inches below 
a small lateral shoot which was 6 inches in length. The leaves of 
such a shoot showed the symptoms of the disease 12 hours before it 
appeared in the young leaves at the tip of the main runner, which 
were 30 inches beyond the point of inoculation. This occurs occa- 
sionally in large vines, but in most cases there is no appreciable dif- 
ference in time between the development of s}Tnptoms at various 
pomts, and the distribution of the vii-us through the plant must be 
comparatively rapid. 

Experiments have shown that the juice of an inoculated plant 
may be infectious 18 to 48 hours previous to the appearance of any 
definite mosaic symptoms and also, as already indicated, that the 
juice from leaves near the point of inoculation may become infec- 



38 



BULLETIN 870, U. S. DEPARTMENT OF AGRICULTURE. 



tious 12 hours earlier than that frorii h'avos at more distant points. 
In those tests cucumber j)lants were inoculated cither at the l)ase or 
tip of the stem, and other ])lants were then successively inoculated 
with fragments of leaf tissue from various j)oints on the plant first 
inoculated. These later inoculations were made every 1 2 or 24 hours. 
The results of this work are given in Tabic XIV. The fact that mo- 
saic diseased plants possess the power of infection before the appear-j 
ance of visible symptoms is a most important factor in its relation taj 
disease dissemination and control. Efforts to control cucumber mo-l 
saic in the held by the eradication of all diseased plants as soon 
they appeared have met with little success, largely because of th( 
fact that a great number of plants were constantly serving as a sourc< 
of infection cluring the period wh(>n their juices were infectious, bu( 
before visible s^onptoms had appeared. 

Table XIV. — Occurrence of mosaic virus in inoculated cucumber plants prior to the 
appearance of visible symptoms. ^ 



Plant 
No, 



Date of first 
inoculation. 



Point of inocu- 
lation. 



Date first 
symptoms 
were noted. 



Part used for secondary 
inoculations. 



Date of first 
successful 
secondary 

inoculation. 



-) 

Time • 
virus , 
was 
present 
before 
first . 
sjTnp- 
toms 
appeared 
(nours). 



July 18,1910 

do 

....do 

....do 

....do 



Aug. 8, 1917 



Tip of stem.. 
Uaseofstcm. 
Tip of stem . . 

Base of stem . 



Tip of stem... | 



Base of stem. 



July 22,1910 
July 24,1916 

do 

....do 

....do 

July 25,1910 

do 

\ug. 15,1917 



Aug. 16,1917 



Leaf at tip of stem 

do 

do 

Leaf at base of stem 

Leaf at tip of stem 

Leaf at base of stem 

Leaf at tip of stem 

Tip leaf of shoot inches 
from poiut of inocula- 
tion. 

Tip leaf of m:un stem 30 
inches from point of 
iuoculatiou. 



July 22,1910 
July 23.1910 

do 

do 

do. 



July 24,1910 

JiUy 23,1910 

Aug. 13, 1917, 

6.45 a. m. 

Aug. 13, 1917, 
4 p. m. 



METHOD OF DISTRIBUTION OF THE VIRUS. 

The distribution of the virus tliroughout the plant is probably 
effected either by dilfusion through the parenchymatous colls or by 
conduction through the vascular elements. While the que-stion is 
diflicult to determine definitely, it seems more probable, in the 
writer's estimation, that the vascular system is most important, 
although diffusion may also be a factor. When inoculation is made 
at the base of the stem, the fii-st symptoms, which invariably appear 
in the young leaves, develop as rapiilh' as when inoculation is made 
directly into these leaves. It thus appeai-s that the distribution is 
rapid, since the young leaf tissue may contain the virus within tliroe 
days after inoculation in sudicient amount to produce infection in 
other plants. It would seem that dilfusion alone would be too slow 
to allow the virus to reach all parts of the plant in as short a time as 



THE MOSAIC DISEASE OF CUCURBITS. 39 

it does when inoculation is made in the stem tissues. The fact 
that the pores of a Chamberland filter can hold back the particles 
of infectious material also leads to a belief that such particles may 
bo colloidal in nature and not likely to pass readily through the 
cell membranes. 

Allard (5) has shown in the inoculation of older leaves of tobacco 
plants that severing the lateral veins from the midrib or cutting the 
base of the midrib itself does not appreciably increase the time 
usually required for the virus to reach the stem and pass to the young 
leaves. He believes that in such cases "multiplication and diffusion 
of the virus from cell to cell, aided perhaps by the fine anastomosing 
lateral veins, would sooner or later allow the virus to reach the 
petiole and pass to the rest of the plant." The writer is inclined to 
believe from the results of work with cucumber aphids that conduc- 
tion through the fine veins is a more important means of transference 
than diffusion. These insects apparently attack only the small veins 
of the leaf, as shown both by observation and by stained microtome 
sections of aphids killed and embedded while still attached to the leaf. 
In all such sections the sucking apparatus of the insect extended 
into the small leaf veins. Further support of this fact is given by 
Woods (31) in his work on stigmonose of carnation, in which he 
shows drawings and photomicrographs which indicate that the aphis 
attacks the vascular elements. Since aphids are the most consist- 
ently successful agency of inoculation for cucurbit mosaic yet dis- 
covered, it seems likely that their high percentage of infection is due 
to the introduction of the virus at a point where it is most rapidly 
carried to all parts of the plant. Stem inoculations made by cutting 
off a petiole and pricking the juices of mosaic cucumber plants into 
the region about the bundles of the leaf trace give about double the 
percentage of infection that occurs when the virus is pricked into 
the stem at random, indicating again that the vascular system is 
concerned. 

The portion of the vascular system especially concerned is still 
rather indefinite, but the work with aphids again furnishes a clue. 
Woods's work before mentioned (31) indicated that the soft bast 
parenchyma cells of the phloem were the ones particularly attacked. 
Other observers have also recorded the same facts, and the \\Titer's 
work along this line, although not extensive, indicates that the 
cucumber aphis punctures the cells of the phloem. If this is true 
it may be possible that the pliloem is at least as important as the 
xylem in the distribution of the virus, since the inoculation work with 
aphids is so universally successful. Beijerinck (6) is of the opinion 
that the phloem is the means of distribution of tobacco mosaic, 
since inoculation of the older leaves first produces symptoms in the 
young leaves, and he therefore concludes that the virus is carried in 
the descending sap flow. 



40 BXTLLKTIN 87f), U. S. DEPARTMENT OV AGRICULTURE. 

All tho nl)<)V(> conclusions arc Tncicly tentative, liowcvcr, particu- 
larly in rcf]jar(l to the rOlc of tho ])hl()em in tho distrihution of tho 
virus. The vascular elements, however, seem to offer the most 
pro])ablo channel for the rapid distribution of infectious material 
through the plant. 

INCUBATION PERIOD. 

Tiio incul)ation period of cucurbit mosaic varies somewhat with 
the age of the plant inoculated. In young cucumber plants which 
ai'o growing rapidly the lirst visil)le signs of the disease may appear 
within 4 to 5 days following inoculation, and the incubation period 
in such plants vnW rarely exceed H or 9 days. In the case of older 
plants which are growing less rapidly, the period of incubation is 
often longer and the first symptoms may not develop for 12 or 141 
days, although these plants generally do not require over 10 days] 
for the appearance of the symptoms. ' 

Tlie incubation period for plants of the same age and vigor of 
growth is quite constant, and where several plants are inoculated at 
the same time those which are infected show the signs of the disease 
within 3 to 4 days after the earliest symptoms appear. Plants 
which do not develop mosaic sj-mptoms \vithin this period usually 
remain healthy. 

The incubation period of the other cucurbits is very nearly the 
same as that of the cucumber and varies within approximately the 
same limits. In the case of large scjuash and gourd })lants the incu- 
bation period is slightly longer, ordinarily, than that of most other 
cucurbits, and usually extends over 12 or 15 days. On the younger 
plants of all species, the first symptoms will normally develop within 
7 to 9 days after inoculation. Tlie general incubation period may be 
said to lie between 4 and 15 daj's, with the average period varying 
from 7 to 12 days for all species. 

MOSAIC TRANSMISSION. 



I 



Many mosaic diseases are of such a nature that field operations, 
in which, healthy and diseased j)lants are handled in succession, 
tend to further disseminate the disease. This has been demonstrated 
by Hunger (15) for tobacco mosaic and is particularly true with the 
mosaic on cucumber, the method of growing and harvesting the crop 
being such as to favor the spread of infection throughout the season. 
As infection may occur whenever the juices of a mosaic plant come in 
contact with slight wounds in healthy vines, any thinning, training, 
or other handling of mosaic and healthy j)lants may jiroduce infec- 
tion. That nieic handling of the ])lants may ri'snlt in disease trans- 
mission has been j)r<>ved by bruising the leaf of a mosaic vine between 
the fingers and then handling the leaves of healthy i)lants in a simi- 



THE MOSAIC DISEASE OF CUCURBITS. 



41 



lar manner, but without crushing them. The results, shown in 
Table XV, indicate the ease with which infection may be brought 
about by this method. 

Table XV. — Infection resulting from successive handling of mosaic and healthy cucumber 

plants. 





Trealmont. 


Number 
of plants. 


Results. 


Date. 


Number 

ofmosaic. 

plants. 


Date 
observed. 


Aug. 24,1910 
Po 

Aug. 11,1917 
Do 

Mar. 12,1018 
Do 


Leaves of diseased and healthy plants handled in suc- 
cession. 

Leaves of two healthy plants handled in succession 
(control). 

Leaves of diseased and healthy plants handled in suc- 
cession. 

Leaves of two healthy plants handled in succession 
(control). 

Leaves of diseased and healthy plants handled in suc- 
cession. 

Leaves of two healthy plants handled in succession 
(conirol). 


5 

4 
6 
6 
10 
8 


3 


4 





Sept. 4,1916 

Do. 
Aug. 30,1917 

Do. 
Mar. 28,1918 

Do. 



TRANSMISSION DUE TO CULTURAL OPERATIONS. 

TRAINING AND REMOVAL OF INTERTWINED PLANTS. 

The evidence reviewed above shows that handling may spread 
infection at any time, but it is more common later in the season 
when the plants have made a heavy growth. As the vines lengthen 
it is necessary constantly to train them back from the center of the 
row, and in picking they are also lifted and pulled about. Tliese opera- 
tions produce many small abrasions, as the adjacent vines are usually 
interlaced, and when a mosaic and healthy plant occur together 
these slight wounds very often result in infection. It is commonly 
observed where few insects are present that the disease spreads 
along the row to the plants adjacent to the first isolated cases of 
infection. That this is due to such means as those mentioned above 
was strikingly demonstrated at Big Rapids, Mich., in 1916. The 
mosaic appeared at several points in a small field, and during the 
remainder of the season, in an effort to control it, all plants showing 
the disease were removed as soon as the first definite symptoms 
were observed. It was noted that, 10 days after the removal of the 
diseased individuals, plants in the same row and immediately adjacent 
to them began to show symptoms of mosaic, the disease progressing 
down the rows in both directions from the original diseased plants. 
In this experiment the vines were removed carefully, with the idea 
of avoiding infection, but slight injuries were unavoidable, and it is 
evident that such infection is constantly occurring throughout the 
season. It is probable that the factor of trichome infection is of 
special importance in cases of tliis sort, since the injuries need be but 
slight to produce infection through this channel. 



I 



42 



BULLETIN 87r>, IT. S. DEPARTMENT OF AGRICULTURE. 



INFECTION UY WAl.KINO ON PLANTS. 



In many cases the ^'inos gi-ow so tliicldy that a certain amount 
of injury from treading on the ])lants during ])i(king is unavoitlnble, 
particularly where tlie rows are narrow. This ])robably has ])roduced 
infection in many cases, one of whicli was noted by IMr.- W. W. Gillx'rt 
in 1915 at Muscatine, Iowa. A party of seAenil persons had crossed 
a field of seed cucumbers where the growth was rank and many 
mosaic plants were present. In so doing, vines were necessarily 
crushed under foot. A field of healthy ])lants in the neighborhood 
was ^'isited soon after. The growth was hca\y in this secontl field also, 
and further tram])ling of the ])lants occurred. Three weeks later 
the ])ath taken through the second field was distinctly marked by 
mosaic infection, wliilo the remainder of the field was still free from 
mosaic. This doubtless was an extreme cjise, but it is likely that 
some infection takes ])hicc in this way in many fields. 



INFECTION UY PICKINfJ. 



In harvesting i)ickling cucumbers, they arc picked every day or 
every other day, in order to secure small fruits. The fruits are 
broken off l)y hand in rapid succession and the juices, which soon 
cover the fingers, furnish an ideal and rapid means of mosaic trans- 
mission throughout the season. This has been demonstrated, a>> 
shotvn in Table XVI, by alternately picking fruits from diseased and 
healthy vines. The fruits were merely broken off Mith the thumb 
nail and no effort was made to produce any greater wouncUng or ad- 
herence of juices to the fingers than is common in field operations. || 

Table "KYI.— Experiments to -prove mosaic infection of cucumber plants due to picking. 





Treatment. 


Number 
of fruits 

picked 
pr 

plant. 


Number 
of plants 
inocu- 
lated. 


Results. 


Dale. 


Number ^ ^ , » 


Aug. 5,1910 

Do 

Oct. 25,1910 

Do 

Nov. 24, 1917 

Do 


Fruits picked from healthy plant after picking 

mosaic fruits. 
Fruits picked from healthy plantsonly(control). 
Fruits ijjekod from healthy plant after picking 

mosaic fruits. 
Fruits picked from liealthyplantsonly (control). 
Fruits picked from hcaltliy plant after picking 

mosaic fruits. 
Fruits picked from healthy plants only(control). 


2 

2 
2 

2 

1 

1 


4 

4 
5 

5 
7 

7 


3 


3 


4 




Aug. 21,1916 

Do. 
Nov. 15,1917 

Do. 
Dec. 8,1917 

Do. 



WOUND.S NECE.'«SAUY FOR CONTACT INFECTION. 

In all the above work, the wound factor must be emphasized, 
since infection lias never been knoNMi to occur wliere diseased and 
liealthy \iuc^s were in undisliirbed contact. Hundreds of cases have 
been observed where only a portion of the plants in a cage would 



THE MOSAIC DISEASE OF CUCURBITS. 



43 



develop mosaic symptoms after inoculation, and these healthy and 
diseased plants would remain intertwined for several weeks without 
further cases of mosaic appearing, no insects being present to cause 
infection. 

INSECT TRANSMISSION. 

While dissemination of mosaic often occurs by the means already 
noted, a large part of the infection in the field is often due to insects 
attacking the cucumber. 

TRANSMISSION BY APHIS GOSSYPH. 

The importance of insects as a source of infection has been demon- 
strated by both Jagger (17) and Doolittle (11) in the case of the melon 
apliis {ApMs gossypii Glover), and field observations have repeatedly 
shown that this insect may be instrumental in the production of 
epidemics of cucumber mosaic. At Hamilton, Mich., in 1914, a 
severe attack of aphids occurred over a wide teriitory during the 
latter part of July, when mosaic was present in a number of fields but 
had not become widely distributed throughout the district. As the 
apliids spread from field to field and increased in numbers, it was 
noted that a severe outbreak of the disease usually occurred about 
ten days after the appearance of the insects. Within a month most 
of the fields about Hamilton showed from 60 to 90 per cent of mosaic 
infection, and the crop was practically destroyed. Similar results of 
aphid infestation were noted in Michigan in 1916 and in Wisconsin 
in 1917, but over less extended areas. 

Table XYII.— Transmission of cucumber mosaic by Aphis gossypii. 





Treatment. 


Source ofaphids. 


Number 
ofaphids 

placed 
on each 

plant. 


Number 
of plants 
inocu- 
lated. 


Results. 


Date. 


Number 

of mosaic 

plants. 


Date ob- 
served. 


Aug. 6,1915 
Aug. 23,1915 

Do.. 

Aug. 24,1915 

Do 

Aug. 30,1915 

Do 


Inoculated 

do 


Mosaic cucumber leaves 

do 


25 to 30 

15 

30 

10 

20 

5 

3 

15lo25 

10 

8 

6 

6 

3 

2 

8 

5 

3 

1 

10 


8 
8 
8 
9 
6 
7 
6 

10 
6 
9 

12 
8 
8 
8 
8 
6 
6 
6 
8 


8 
8 

9 

7 
6 

6 
9 
9 
7 
6 
2 

5 
1 




Aug. 21,1915 
Sept. 3,1915 


Control 

Inoculated 

Control 

Inoculated 

do 


Healthy cucumber leaves. . 

Mosaic cucumber leaves 

Healthy cucumber leaves. . 

Mosaic cucumber leaves 

do 


Do. 
Do. 
Do. 
Sept. 10,1915 
Do. 


Do 

Aug. 4,1916 
Aug. 10,1916 
Aug. 5,1917 
Aug. 17,1917 

Do 


Control 

Inoculated 

do 


Healthy cucumber leaves. . 

Mosaic cucumber leaves 

do 


Do. 
Aug. 14,1916 
Aug. 18,1916 


do .. 


.. .do 


Aug. 16,1917 


do.... 


...do 


Aug. 29,1917 


do 


do 


Do 


do .. 


.. .do 


Do. 


Do 

Sept. 1,1917 
Do 


Control 

Inoculated 

do 


Healthy cucumber leaves. . 

Mosaic cucumber leaves 

.do 


Do. 

Sept. 10, 1917 
Do. 


Do 


do 


do 


Do. 


Do 


Control 


Healthy cucumber leaves . . 


Do. 



The agency of the insect in transmitting mosaic has been proved 
by experiments in which aphids taken from a mosaic cucumber plant 
were placed on healthy plants imder cages. In nearly all cases, 



44 BULLETIN 87f>, U. S. DEPARTMENT OF AGRICULTURE. 

tliis has resultod in tlio infoction of all tlio ])lants on wliich the insect 
woro ])Iaco(l (PI. II, /?). Aphids transfcrrcMl from one healthy planl 
to another liave never ])ro(luce(l the disease. I 

The liif^h ])er(enta<;je of mosaic infection obtained from inoculatioi 
by means of a])liids from diseiised ])lants (Table XVJI) is probabli 
due to the fact that, bein«^ a sucking insect, it introduces the \inii 
into tliose tissues which will cUstribute it most rapidly throughouj 
the ])lant. The number of a])liids necessary to produce infectioa 
is small, infection liaving occuiTcd where only three of the in.sect^ 
were placed on each plant, but where only one to three individuals 
are used the ]>ercentage of infection is often lowered. 

TRAN'SMISSION BY DIABKOTKA VITTATA. 

Although the cucumber aphis may be responsible for severe epil 
demies of mosaic, the striped cucumber beetle {Diahrotica vitiai 
Fabr.) is probabl)^ the most important insect agent in the transmis- 
sion of the disease. AMiilc the aphis can cause extensive and rapid 
mosaic dissemination when present in large numbers, it appears 
only at intervals in most districts and is then often confined to a few 
fields. The striped beetle, however, is common to most cucumber- 
growing sections and usually occurs in considerable numbers tluough- 
out the season. It is a very active and voracious insect and attacks 
practically all parts of the plant, but more particularly the leaves. 
While apliids will nearly always j)roducc infection when transferred 
from mosiac to healthy plants, the percentage of infection is much 
lower in the case of the beetle. Being a chewing insect, it trans- 
mits the disease only when mosaic plant juices are carried on its 
mouth parts to wounds in healthy plants and are then distributed 
throughout the tissues. ]Man3' of these insect wounds are slight, 
however, and are often located in the blossoms and at other points 
which are not favorable to the rapid distribution of the virus in the 
plant, so that it is probable that (lr}nng out often prevents infection. 
This low percentage of infection is offset by the fact that the beetles 
are immerous and active, constantly feeding first on one plant, then 
on another, ^\■\{\\ the result that the chances for infection are many. 
In localities where beetles have been numerous the disease lias spread 
very rapidly and usually over a wide territory, as the msect often 
travels considerable distances. 

i}xj)erimental work has given definite proof of the agency of beetles 
in mosaic transmission. Beetles captured directly from mosaic 
Anncrt and placed on healthy plants under cages have produced the 
disease in numerous cases. I'sually a considerable number of in- 
sects were placed in each cage, owing to the lower chances of infection 
before iiKMitioncil, but the numbers were no <:reatcr than were com- 



THE MOSAIC DISEASE OF CUCURBITS. 



45 



monly found on an equal number of plants in the same field. Checks 
were not run with the insects, as in field experiments it was impossible 
to determine with certainty whether or not the beetles had previously 
fed on mosaic vines. The results of these tests, shown in Table 
XVIII, indicate that the beetle must be an important agent of 
transmission in most fields. 

Table XVIII. — Transmission of cucurbit mosaic bt/ caging Diabrotica vittata over 
healthy cucumber plants, as shown by expcrinienis made at Big Rapids, Mich., and 
Madison. Wis. 





Num- 
ber of 
bee- 
tles 
per 
cage. 


Source of the l^eelles. 


Method of infec- 
tion of beetles. 


Num- 
ber of 
plants 
per 
cage. 


Total 
num- 
ber of 
plants. 


Results. 


Location and date 
beetles were caged. 


Num- 
ber of 
mosaic 
planets. 


Date last 
observed. 


Big Rapids, Mich.: 
July 28, 1916... 
Aug. 4,1916.... 

Aug. 5,1916.... 
Aug. 11,1916... 
Aug. 14,1916... 

Madison, Wis.: 
Aug. 11,1917... 
Do 


8 
12 

8 
10 

8 

35 
32 

28 
30 
30 
30 
30 
30 
20 
22 
27 
100 

40 
38 
45 


Madison, Wis 

Big Rapids , Mich . . . 

.do 


Natural infection.. 
Fed on mosaic 

plants in cage. 
do 


2 
2 

3 
3 
2 

8 
4 
7 
9 
7 
5 
5 
6 
6 
5 
6 
40 

IS 
15 
15 


16 

2 

12 
12 
10 

8 
4 
7 
9 
7 
5 
5 
6 
6 
5 
6 
40 

IS 
15 
20 


3 
1 

3 

4 

3 
1 
3 
5 
3 
1 
3 
2 
1 
2 
1 







Aug. 12,1916 
Aug. 17,1916 

Aug. 21,1916 


Madison, Wis 

Big Rapids, Mich. .. 

Madison, Wis 

do 


Natural infection.. 

Fed on mosaic 

plants in cage. 

Natural infection.. 
.. .do 


Sept. 5, 1916 
Do. 

Aug. 21,1917 
Do. 


Do 


do 


do 


Do. 


Do 


.do 


do 


Do. 


Aug. 23,1917.. 
Do 


do . 


. . .do 


Sept. 4,1917 


.do.... 


do 


Do. 


Do 


.do 


do 


Do. 


Do 


do... 


... .do 


Do. 


Sept. 1. 1817... 
Do 


.do 


do 


Sept. 12, 1917 


.do 


do 


Do. ' 


Do 


do 

. .do 


do 


Do. 


Oct. 15, 1917 


(Cucumber vines 

killed by frost.) 

do 


Oct. 30,1917 


Oct. 22 1917 


do.... 


Nov. 17,1917 


Oct 27 1917 


do . 


.do 


Nov. 21,1917 


Nov 10 1917 


do.. .. 


do 


Do. 











It wdll be noted that no beetles collected on or after October 15, 
1917, produced infection. All cucumber plants were killed by frost 
about September 25, and all beetles collected in cucumber fields 
after that date had fed on other plants or on squash and cucumber 
fruits which still lay in the fields and had not fed on mosaic cucumber 
vines for three or more weeks prior to collection. In those tests 
plants were placed m a large cage in the greenhouse w^iich held 40 
plants in small pots, and the beetles were allowed to feed on the young 
cucumber plants for several days. The results indicate that this 
insect can transmit the disease for only a short time after feeding on 
mosaic plants. 

UIABROTICA DUODECIMPUNCTATA AS A CARRIER OF THE DISEASE. 

The 12-spotted cucumber beetle {Biahrotica duodedmpundaia 
Oliv.) is also an agent of transmission of cucurbit mosaic in the 
field. This hisect is so similar to the striped cucumber beetle that 



I 



46 



BULLETIN 875), V. S. DEPARTMENT OF ACRK'ULTl'hK. 



all stutonu'iits rt'pirdiiig one insect Avill pro})nl>ly api)!^ cquiilly well 
to the other. The work with IHahrotica duode dm punctata was done 
in the same maimer as that wilii />. littata, and tho results were 
similar, as is shown in Table XIX. 

Taui.k XlX.— TrdHsmmion of cucumber mosaic bij Diahrolini ihifxhciinpiiminlii in 
experiments at Madison, Wis. 



DatGl>ee(lps 
were caged. 



AiiR. 16,1917 

Do 

Do 

Aug. 21,1U17 

Do 

Do 

Do 

Do 



Number 

of 

beetles 

per 

cage. 



Source of lK>ctIps. 



Madison, \Vis. 

....do 

....do 

....do 

....do 

....do 

....do 

....do 



Method of infection of 
I)cctles. 



Natural infection. 

do 

do 

do 

do 

do 

do 

do 



Number 
of plants 
per cage. 



Results. 



Number p , , , , 



Aug. 30,1917 

Do. 

Do. 
Sept. 20,1917 

Do. 

Do. 

Do. 

Do. 



TRANSMI.SSION UY OTHER CUCUMBER INSECTS. 

It is quite possible that other insects attacking the cucumber may 
also transmit the disease, but they are probably of less importance.. 
Tho tarnished plant bug (Lygus 2>ratensis L.), being a sucking msectJI 
very probably carries the disease to some extent, but the few testal 
made with this insect have given negative results. Tlirips (ThripS"^ 
tabaci Lhid.), the red spider {Tetranych'us teJarins L.), and the white 
fly (Aleyrodes vaporanomm Westw.) have shown no indications ofi 
behig carriers of the disease, although greenhouse observations haveJ 
included a largo number of cjises where the insects were constantlyl 
feeding on adjacent healthy and mosaic ])lants. This is in agreo^ 
ment with the work ofAllard (5), who found that neither the white" 
fly nor the red spider acted as carriei-s of tobacco mosaic in the 
greenhouse. 

TKANSMISHIOM UY IJEi;S. 

Tlio cucumber and other cucurbits which blossom heavily thnmgh- 
out the season are constantly frequented by great numbers of bees. 
These insects might therefore be important agents in disease dis- 
semination, ])rovi(led they carry the infection. However, tho evi- 
dence thus far secured indicates that little, if any, infection is so 
carried. To test this ])()ssibility bees were taken directly from the 
blossoms of mosaic plants and then caged with healthy plants for 
some time. Infection has never been known to occur, altiiough 
trials have been made with 53 plants. As stated on page 33, all 
attempts at artificial inoculation through the blossoms have so far 
failed, and it is likely that if infection occurs at this pomt it is 



THE MOSAIC DISEASE OF CUCURBITS. 47 

rather rare. Field observations, moreover, give little support to the 
theory that bees are a factor in mosaic dissemination, since in many 
cases where few other insects were present the spread of the disease 
would in all probability still have been rapid if the bee were a carrier, 
whereas in such cases the increase of infection was relatively slow. 

POLLEN AS A CARRIER OF INFECTION. 

The work with bees has also brought up the question of possible 
infection through the fertilization of flowers of healthy plants with 
pollen from the blossoms of mosaic plants. This has been tested by 
artificial fertilization of healthy cucumber flowers with pollen from 
mosaic vines, the operation being performed with a camel's-hair 
brush to avoid wounding the flower parts. Although many fruits 
were thus produced, no mosaic infection has ever been found. In 
these experiments care was taken to protect the blossoms from outside 
pollen and to make certain that the fruits set were pollinated only 
with pollen from mosaic plants. 

METHOD OF OVERWINTERING. 

FIELD OBSERVATIONS. 

Since the factors concerned in the field transmission of the disease 
are sufficiently well established to account in large measure for its 
rapid dissemination, the problem of its overwintering and reappear- 
ance each year is perhaps most important, aside from the actual 
cause, in relation to possible methods of control. 

As before stated, the origin of the disease seems unrelated to soil or 
climatic conditions, and its appearance is dependent on some specific 
infection. For the past four years it has appeared in Michigan, Indiana, 
and Wisconsin at approximately the same time, the first reports of 
its occurrence each year having been somewhere between July 7 and 
July 20, at a time when the plants were from 5 to 6 weeks old and 
had from 8 to 10 leaves. Infection of very young plants is rare 
and has usually appeared in cases where the seed was planted very 
late in the season, at a time when infection was already present on 
older plants in the same field. 

The regularity in the appearance of the disease extends to most 
localities in the States mentioned, and a survey of the cucumber- 
growing districts of Michigan or Wisconsin about the middle of July 
usually shows the disease developing almost simultaneously in most 
localities where it was severe the year previous. In many cases the 
first infections are found on a few plants scattered tlu-ough the field, 
or perhaps on plants in a single center, including six to eight vines 
within a short distance of one another, and from these it s])reads 
gradually to neighboring plants. This manner of appearance and 



48 BULL.KTIN 87<>, U. S. DKPARTMKNT OF AGRICULTURK. 



, 



subsoqiiont dovoiopincnt is most roimnon, but in some instnnros the 
tlis(>as(> has suddenly dcvolopod in ('j)i])liytotic foiTn, u striking ox- 
lunplo of which occurred at Madison, Wis., in 101 (>, followed bj 
a second and more spvcro case in 1917. During 191G the writer waj 
not at >radison, but Dr. M. W. Crai'dner reported that the disease 
appeared early in July and in a short time in many fields had affected 
'2r^ to 50 per cent of the plants. 

During 1917, fre(iuent observations were made on the experimental 
fields at Madison from the beginning of the season. The fields under 
observation consisted of six plats within a radius of 1^ miles and 
varied in size from one-half to 1^ acres. These were practically the 
only large cucurbit fields in the locality, Avith the exception of a few 
plantings of muskmelon and squash. T'^p to July 19 no mosaic had 
aii])eared on any cultivated cucurbits in the locality so far as known. 
On this date, however, two of the six plats showed a number of cases 
of mosaic m its earliest stages. On July 21 definite sj^mptoms of 
mosjiic were noted on 25 per cent of the plants in these two plats and 
on a few plants in each of the remaining four plats. On eTuly 20 a 
least 50 per cent of the plants in the fii"st two plats and from 3 to 1 
per cent of the plants in the other plats were affected. A survey o; 
gardens in the vicinity also showed a few cases of the dise^use o 
July 2(3. The plants noted between July 19 and 2G were all in th 
early stages of the disease, and so far as could be determined all the? 
infection must have occurred within seven or eight days. Such a 
sudden development of the disease would seem to indicate that 
some agent of dissemination was present which produced rapid and 
widespread infection at a definite period. | 

The most j)robable factors in overwintering would seem to be: 

(1) .Soil containing refuse from mosaic vines of previous seiisons; 

(2) the use of seed from diseased plants; (3) possible wild cucurbit 
hosts; (4) the presence of an identical disease on plants of another 
family; or (5) some insect agency. Most of the work has therefore 
been conducted with these possibilities in juind. 



SOIL AS A SOURCE OF INFECTION. 



Tt has been shown that the expressed juices of mosaic ]dants 
remain infectious for only a short period and that ilried tissues of 
such plants have never shown the presence of the virus. This indi- 
cates that soil containing tlie remains of mosaic plants is not likely 
to j)rove a sourc-e of infection to further crops. 



(iUEKNIIOrSK KXI'KUIMKNTS. 



During (li{> winter of 191(», 40 ])lants were grown in the gi'eenliouse 
in soil from a field at IljOnilton, Mich., where mosaic had been seri- 
ous foi- tiiree successive yeai"s. The plniils W(>re transplanted to 



THE MOSAIC DISEASE OF CUCUEBITS. 49 

insure root injury as a point of possible infection. As controls, 40 
plants were transplanted into the same soil after it had been steril- 
ized with steam at 15 pounds' pressure for three hours. Plants 
grown in this soil, whether sterilized or not, remained healthy in all 
cases. 

During the month of August, 1910, a large number of mosaic 
cucumber vines were buried about G inches deep in sandy loam at 
Big Kapids, Mich., and left until the following year. In July, 1917, 
some of this soil containing the remains of the mosaic vines was 
taken to Madison, Wis., and used in the following experiments: 

(1) Cucumbers were gi'own in the soil directly from seed, and small seedlings were 
also transplanted into pots of the same soil. In each case, 25 plants were used, 
together with 30 check plants grown in the same soil after steam sterilization for thi'ee 
hours at 20 poxmds' pressure. All plants remained healthy. 

(2) A 50-gram portion of the soil, containing a number of fragments of dried stems 
and leaves, was mixed with 50 c. c. of sterile distilled water and allowed to stand for 
12 hours. This extract was then filtered and the filtrate used to inoculate 10 healthy 
cucumber plants, inoculations being made into stems and young leaves. No infection 
occurred on any of the plants thus inoculated. 

(3) The remains of the vines present in the soil were removed, ground through a 
meat chopper, and enough sterile distilled water added to soak the material into a 
soft mass. Fragments of this were then inserted in incisions in the roots and stems of 
10 healthy cucumber plants, but no infection resulted. 

FIELD TESTS WITH CAGED PLANTS. 

While this work gave only negative results, more conclusive series 
of experiments were conducted in the field during the summer seasons 
of 1915, 1916, and 1917. Observations during 1914 at Hamilton, 
Mich., where mosaic had been severe for some time, showed that the 
disease seemed to occm- as early and severely on new land as on fields 
where mosaic cucumbers had been grown for two or three years in 
succession. An attempt was made therefore, to determine whether 
fields that had previously grown mosaic plants furnished a source of 
infection for crops the following season. 

Hamilton, Ilich., 1915. — One of the fields wliich had been under 
observation for two years was selected for work in 1915. The plat 
was one-eighth of an acre in size, had grown cucumbers for three con- 
secutive years, and nearly every plant had been diseased each year. 
This plat was again planted to cucumbers in 1915, and as soon as the 
plants appeared 18 cheesecloth cages were distributed over it, each 
cage covering two plants. The cages were lifted only once during 
the season, to allow weeding and thinning, and every precaution was 
taken to avoid outside infection. Mosaic appeared in the plat about 
Jul}^ 20, and on September 6 every uncaged plant showed mosaic 
symptoms. The cages were lifted on this date and the entire 36 
plants were found free from disease. 
185118°— 20 4 



50 BULLETIN 871), U. S, DEPARTMENT OF AGRICULTURE. 

Hamilton, Mich., 1.916. — The snmo pint was usod in 1016 and 30 
cages sot out on July 10, covorini; GO plants. Tlio un(a<;;o(l plants 
bocamo afToctod oarly in the soason and on August 22 each of them 
showed the disease in severe form. The caged vinc^ liad growTi so 
raj)idly that it was necessary to remove tho cages at this time, and it 
was found that all had remained healthy. 

Biff liajnds, Mich., 1916. — A test similar to that at Hamilton was 
conducted at Big Rapids, Mich., on a quarter-acre plat that had 
been badly diseased the previous year. Thirty-five cages were pul^ 
dcmn on June 20, live on Jime 29, and five on July 7. However, the 
disease was not severe in the imcagcd portions of the field, and only 
10 per cent of the plants were found to have mosaic at the end of tho 
season, so that tho results were not as conclusive as in the other tests. 
All caged plants remained healthy throughout the season. 

Madison, Wis., 1917. — In 1917 the work was transferred to Madi- 
son, Wis, and the experiment was repeated on a f-acro plat where all 
plants had mosaic in 1916. 

The cages in this case were set out on Juno 26, as soon as the seed 
germinated, 18 cages being used, covering 36 plants. Beetles wer 
pre«sont in tho field in gi'eat numbers after July 10, and a ft 
appeared in some of the cages after a heavy rain which had washe 
the earth away from the edges of the cages. The plants in these 
cages were removed on July 9 and new seed planted. The cages 
were watched carefully; no more insects appeared until August 18, 
when l)eotlos were found working in nearly every cage, but as there 
scwmed to be no means of entry from tho outside it was suggested 
})y entomologists who were consulted that they were from tho second 
])rood and had emerged from tho soil. No effort was made to 
remove them, and the cages were left until September 13. The 
uncaged plants had been very badly infected with mosaic early in 
the season, 25 per cent of the plants developing tho disease witliin 
five days after July 19, when tho first cases of mosaic were foimd. 
By September 13, when all the uncaged plants had been infected 
for some time and the vines were much stunted, tho caged plants 
still appeared perfectly healthy and normal with the exception of 
injuries from the beetles noted above. (PI. IX, B.) 

During the same season an additional test was carried on in a small 
garden belonging to the department of plant pathology. Here 12 
cages were used to cover 24 vines which were transplanted from the 
greenhouse early in June. Fifty additional plants were transplanted 
near by and 100 more were grown from seed, both lots beiug left 
uncagi^d. All the uncaged vines became diseased by August 20, and 
when the cag<s were remov(Hl on August 29 all tho caged plants 
proved perfectly normal and healthy. 



THE MOSAIC DISEASE OF CUCUEBITS. 51 

The results of these experiments furnish very definite proof that 
the soil plays little, if any, part in the overwintering of cucumber 
mosaic. The fact that the virus has not been fomid in the tissues 
of old and decayed vines, coupled with the fact that vines protected 
from insects can be kept free from mosaic year after year on soils 
where the disease has been present for several consecutive seasons, 
seems to exclude the soil from consideration as a means of over- 
wintering. 

SEED AS A MEANS OF OVERWINTERING. 

The problem of the transmission of mosaic diseases through the 
seed from infected plants has received much attention from various 
writers in the case of tobacco and tomato. No definite evidence of 
seed transmission in the case of tobacco has ever been obtained, 
although Allard (3) showed the presence of the virus in the flower 
parts, capsules, and mature seed. In the case of tomato mosaic. 
Miss Westerdijk (30) reported a possible example of the transmission 
of the infection by seed. Reddick and Stewart (24) have also stated 
that bean mosaic is seed borne. The question of seed transmission 
of certain mosaic diseases is therefore still an open one. 

FIELD OBSERVATIONS. 

Certain field observations on cucumber mosaic, together with the 
fact that the disease is prevalent in seed-growing districts, have 
made it of great importance to determine definitely whether the 
disease may be carried over on the seed each year. 

As in the case of tobacco mosaic, the fruits of mosaic cucumber 
plants contain the infective principle, and their juices will produce 
infection. As far as can be determined, these juices remain in- 
fectious until the seeds are nearly mature, but the presence of the 
virus in the immature seed or in the tissues surroundmg it can 
hardly be definitely proved, since the thick flesh surroimding the 
seeds makes it difficult to remove certain portions of the fruit, including 
traces of the juice from other cells. As stated on page 36, inoculations 
made from mature seed have never shown the presence of the virus, 
but it is possible that the seed may contain the virus in rare cases 
and perhaps afford an opportunity for overwintering the disease in 
this way. 

The opportunity for seed infection is present, however, if such 
infection be possible, since the disease is prevalent in many of the 
seed-growing districts. Much of the cucumber seed, especial h' of 
pickling cuciunbers, is grown in the vicinity of Muscatine, Iowa, 
Rocky Ford, Colo., and to some extent in Michigan and Ohio. The 
mosaic disease has been fomid to occur very connnonly in those 
districts, and in handhng the crop little or no effort is made to cull 
out fruits from mosaic plants. Since many of these fruits are nearly 



52 BULLETIN 879, U. S. DEPARTMENT OF AGRICULTURE. 

normal in size and ai)i)oaran('o at maturity, a groat nmnl)t'r are 
included with the seed crop each year and there is no doubt that 
much of the seed stock furnished to gi'owers contains many seeds 
from mosaic vines. 

louring the last four years a number of cases of infection have been 
obsened which gave some su])port to the theoiy of transmission of 
mosaic through the seed. At Hamilton, Mich., m 1914, careful field 
obseiA' atiofis had been made dming the first 10 days of July, and but 
one field was found in which mosaic was ])resent. On July 12, how- 
ever, at a distance of 1 miles from the to\N'n, a small patch of cucumbers 
was found in which 25 per cent of the plants were diseased. The 
plants were groNsiiig on land cleared the year i)revious and Avere 
smTounded by a belt of woods. No other cucumbers were within 2 
miles, and the only mosaic plants foimd up to that time were 4 miles 
away. The plants showed no insect injury, and it was diihcult to 
explain the origin of such early infection except by seed transmission. 

Another striking case occurred at Big Rapids, Mich., in 1915. 
Tliis locaUty was practically free from mosaic, and none was found 
mitil July 24. On this date one mosaic plant was fomid in the center 
of a 2-acre experimental plat of cucumbers. The land had neverj 
gro%\Ti cucumbei"s before, and the few additional fields in the vicinity 
were free from the disease. The plant found was immediately covered 
with a cheesecloth cage, to prevent the spread of infection, and no 
further cases appeared during the summer with the exception of 
plants inoculated mider cages. 

Several other instances of infection occurring early in the season in 
isolated fields have been noted, and a few similar cases have api)eared 
in the gi'eenhouse, where the mosaic suddenly develo})ed during the 
winter in houses that had previously been free from it and were so 
isolated that outside infection seemed milikely. 

McClintock (22) reported obsers'ations made at the Vii^inia Truck 
Experiment Station at Norfolk which he beheves may indicate the 
transmission of cucumber mosaic by the seed. Plants from the 
greenhouse were transplanted to coldframes in the field, in some cases 
on sterilized soil. Considerable mosaic was later fomid on these 
plants, and no evidence of insects or insect uijury was noted. 

All field obsen'ations of the type mentioned arc open to objection, 
however, in so far as they are taken as definite cAadence of the trans- 
mission of the disease by the seed. So many somces of infection 
exist and their detection is often so dillicult that infection may occur 
in isolated localities from sources that are easily overlooked. In- 
sects, such as striped beetles, may travel considerable distances, 
especially in high winds, and might infect a few i)lants and disai)i>ear 
soon after. Mosaic ])lants of (lie wild cucuiiibcr {Micrampehs lohata) 
may also be ]>reseii(, being often oviMlooked l)ocause of their loca- 



THE MOSAIC DISEASE OF CUCURBITS. 5S 

tion in out-of-the-way places. All such infection is so easily possible 
and so difhcult to trace that field observations are always subject 
to doubt as proof of seed transmission of the disease. 

TESTS WITH SEED FROM MOSAIC PLANTS. 

In order to obtain more definite data in regard to seed transmission, 
a number of tests have been made with seed saved from mosaic 
plants. These trials were conducted both in the greenhouse and in 
the field, the field tests being at Big Rapids, Mich. 

Tests of 1915. — Seed collected from mosaic plants at Hamilton, 
Mich., during the fall of 1914 was tested in the greenhouse of the 
department of botany at the Michigan Agricultural College at East 
Lansing, Mich., during the winter of 1914-15. The seed from indi- 
vidual plants was not kept separate in these trials, most of it bemg 
very immature, and only 90 plants were obtained from the entire lot. 
These were grown in pots on sterilized soil for five weeks, but no signs 
of mosaic developed. 

Tests of 1916. — During the fall of 1915, seed was collected from 
mature fruits of mosaic cucumber plants, using only those fruits 
which actually showed evidence of the mottling and deformity charac- 
teristic of the disease. The seed from each fruit was removed sepa- 
rately, and in planting these separate lots w^ere kept distinct. 
' A preliminary test was made during the winter of 1915-16 in the 
greenhouse at East Lansing, Mich., 500 plants being grown in pots 
on sterilized soil at a temperature of 28° C. (82° F.) and kept under 
close observation for seven weeks. No evidence of mosaic appeared 
on any of the plants during this trial. 

An extensive field experiment was made during the siunmer of 
1916 at Big Rapids, Mich., on land which had not grown cucumbers 
previously and in a district nearly free from the disease. The seed 
was from the lot used in the greenhouse test during the winter and 
represented seed collected from mosaic plants in Michigan, Indiana, 
Wisconsin, Iowa, and Ontario, Canada. The seeds from each fruit 
were planted in separate rows 3 feet apart, with the seeds about 4 to 
8 inches apart in the row. Approximately 100 fruits were represented , 
which gave about 4,500 plants for observation. The germination 
was very mieven, but this seemed to result from the immaturity of 
some of the seed rather than from any effect of the disease, since all 
seed which germinated produced plants of healthy appearance. 
Observations were made practically every day in the early part of 
the season and at least once every two days in August and September. 
All plants remained healthy and normal in appearance and no disease 
appeared in the adjoinnig plats until July 28. On this date mosaic 
appeared on micaged plants in a plat close to the seed test, tlu'ough 
the accidental escape of aphids used in other experiments. In spite 



54 BULLKTIN 871), U. S. DEPARTMENT OF AGRICULTURE. 

of (haslif ofTorts for its eradication, the disease si)rea(l <;rudually 
through the phit until it reached the rows adjoining tiie seed test 
where it first appeared on August 8. After this date, it worked 
slowly through tlie plat, although all mosaic vines were removed as 
soon as noted. The plants had remained healthy for seven weeks, 
however, and the disease which appeared was so clearly a result of 
outside infection that the evidence of seed transmission may safely 
he called negative. 

Tests of 1917. — \i\ 1917 a further trial of mosaic seed was again 
conducted at Big Rapids, Mich., using seed collected from Michigan, 
Indiana, and Wisconsin. In this work the seed from 119 fruits was 
planted on new land, located at some distance from that used in 
former experiments and on considerably higlier ground. 

The plat was about 1 acre in size and was between two other blocks 
of cucurbits, principally cucumbers. No other experiments with 
mosaic were made during the season in any of the plats. About 
5,500 plants were under observation and 1,150 of these were covered 
with cheesecloth cages to prevent accidental infection by insects. 
Use was made of 250 cages, each covering from 3 to 15 plants. They 
WQTQ set out as soon as the seed was planted and were not lifted until 
late in July, except where repairs were necessary. The writer was 
absent from Big Rapids most of the time, and the planting and early 
inspection work was done under the supervision of Mr. W. W. Gilbert. 
The plants were left un thinned as long as possible and were inspected 
at frequent intervals. The writer examined the plants, beginning 
July 30, and lifted all cages for the purpose of inspection. Great 
care was taken to avoid outside infection from insects, and the cages 
were replaced at once. A few striped beetles were present in the field, 
but only one cage was found to contain insects. All uncaged plants 
in the seed test and in the adjoining plats were free from mosaic, and 
all caged plants were healthy with a single exception. This plant 
was abnormally dwarfed, its lower leaf was j^oUow and wilted, and 
the four other leaves were peculiarly dark green in color and showed 
shght indications of a yellow mottling. (PL I, A.) All of the remain- 
ing j)lants which were in the same cage were of normal size and appear- 
ance, without the least trace of insect injury. This cage had no de- 
fects in its covering and had not been lifted after the seed was planted. 

The writer again visited the field on August 18 and inspected the 
plants. No case of mosaic was fomid in the plat with the exception 
of the suspected plant found on July 30. The appearance of this 
was so unmistakably mosaic that 12 healthy plants were inoculated 
from the juice of one of its leaves and covered with cages. Most of 
these develoj)e(l typical mosaic symptoms on leaves and fruits, and 
there is no doubt that the plant in question was mosaic. Mr. Gilbert 
reported that no further cases of mosaic a|)peare(l on cucuinbei's in 



'^1 



THE MOSAIC DISEASE OF CUCURBITS. 55 

any plat during the season, but two plants of Hybrid Casaba melon 
in another plat did later develop typical mosaic symptoms. These 
plants were pulled as soon as found, and no other cases occurred. 
The conditions at Big Rapids during 1917 were unusually favorable 
for a test of this kind, since the district was practically free from 
mosaic, and insects were not unusually niunerous. The single case 
of mosaic that occurred in the seed test can hardly be attributed to 
outside agencies, since soil and insects were eliminated almost beyond 
question and there was apparently no source of infection in the dis- 
trict adjacent to the experimental field. Furthermore, the very 
nature of the disease symptoms was such as might be expected in 
cases of seedling infection, the plant being much dwarfed and the 
first leaf showing signs of having been affected almost as soon as it 
appeared. 

Another case has oocmTed which is very similar to the above and 
lends support to the theory of seed as a means of overwinterhig. 
Seed was planted m the greenhouses of the plant-pathology depart- 
ment of the Univ^ersity of Wisconsm in the fall of 1917, immediately 
after the houses had been thoroughh^ cleaned and fumigated and 
fresh soil placed in the benches. The plantmg was made on Septem- 
ber 12, using ordmary commercial cucumber seed, and the plants 
were under observation daily after they appeared. On Septem- 
ber 23, one plant out of 65 showed defuiite mosiac mfection, the 
cotyledons were yellowed and wilted, and the fu'st true leaf, which 
was v^ery small, was mottled and distorted. (PL X, A.) The plant 
was proved to be infected by mosiac by the successful moculation 
of several healthy plants from it. It was kept for several Aveeks in a 
separate house for observation and later developed the same peculiar 
\\Tinklmg and yeliowmg observed m the case at Big Rapids. It 
remained much stunted and deformed. All the other plants grew 
vigorously and showed no signs of the disease. (PI. X, B.) 

In this case the factor of insect transmission can not be enthely 
eliminated, smce insects were present m the near-by fields until some 
time m November, but the facts that no injury was visible on any 
plant in the house and no insects could be found partly removed this 
objection, since slight injury of this kind is usually very noticeal)le 
on young seedlmgs. 

The results so far, tJierefore, show that out of 10,000 plants groAni 
from seed from mosiac vmes, only one has developed a case of mosaic 
that could not definitely be attributed to outside hifeotion. Together 
with this is the case above noted, which appeai-ed in the greenhouse 
mider conditions that rendered outside hifection very unhkely. The 
shigle case of hifection w^hich appeared in the open field at Big Rapids, 
Mich., m 1915, is most easily explahied on the basis of seed trans- 
mission, shice no other mfection developed in the field aside from that 
resulting from artificial hioculation. The field data aside from tliis 



56 BULLETIN 870, U. S. DEPARTMENT OF AGRICULTURE. 

aro of rather doubtful value as evidenee of tno oecurrenee of the | 
disease from the seed. Whiles these data seem overwhelmingly' 
negative at first glance, the single case of mosaic which developed 
from the seod of mosaic vines was of sueli a charneter that it is 
difiicult to cx]>lain its aj)])earance on any basis other than tliat of ' 
seed transmission. It is dou}>tful whether cucumber mosaic is seed 
borne to any considerable extent, but it seeuLs possil)l(^ that it may 
occur in rai'e cases, and the ri«ults so far obtained have l(>ft tlie ])rol)- 
lem still open, warrajituig further mvestigation. In general, the 
(Usease a])peai's every year in most infested districts, and in a 
manner that would j)reclude any belief in seed transmission as the 
sole cause of its outbreak. In districts where the disease had not 
a]>peared, however, a single case of seed transmission in several 
thousand plants would be suOicient to hitroduce the disease, and the 
many agents of dissemination present Avould hisure its rapid spread 
to other jilants and fields. 

INSECTS IN RELATION TO OVERWINTERING. 

The amount of cucumber mosaic present each year in \arious 
localities has frequently been in direct relation to the numl)er of 
cucum])er insects in the fields. This would be expected in view of 
their im})ortance as agents of dissemination. It has been found, 
also, that the first appearance of the disease in many localities seems 
closely related to the number of insects present, as far as tlio amount 
of the early infection is conoenied. During 1916 and 1917 there 
was an unusually small number of cucum})er bisects in southwestern 
Michigan, and the disease aj^pcared hi a much less severe form than 
in ])revious seasons. Durhig the same period, cucumber hisects at 
MaiHson, Wis., and hi adjacent localities were present in unusually 
large numbers hi June and July, particularly the Diabrotica s])ecios. 
The disease developed there with great severity both years, several 
fields hi 1917 having 15 to 25 per cent of the plants affected withhi 
five days from the tune the first case was noted. These observations 
led to tlie bohef that hisects might bear some part in tlie overwintering 
of mosaic. 

THE RELATION OF APHIDS TO OVERWINTERING. 

During 1915 and 1916 an attempt was made to overwijiter a])hids 
takcin from mosaic vhies, but no insects a])]>eared in the s])ring and, 
as little is known of the method by which this insect passes the winter, 
the work was finally abandoned. As tho cucumber aj)liis (Aphis 
goHfiHini) has always a|)])eared rather late in the season add not until 
some time after the disease developed, it is ])n)])able that this insect 
is not a factor in tlie lirst ap])earance of tlie disease. This is further 
incHcated by the fa<-t that a]>hi(ls are not ])resent every yeai'in locali- 
ties where l\w, diseju>e appears each season. 



THE MOSAIC DISEASE OF CUCURBITS. 57 

THE RELATIONT OF STRIPED CUCUMBER BEETLES TO OVERWINTERING. 

The work of Rand (28) on the Diabrotioa species as possible carriers 
of the bacterial wilt organism {Bacillus traelieiphilus Smith) has 
already shown that these insects are an important factor in the over- 
wintermg of one of the diseases of cucurbits and strengthened the 
hypothesis that they might bear the same relation to mosaic over- 
whitering- The striped cucumber beetle (Diahrotica vittata) and the 
12-spotted beetle (D. duodecimpunctaia) both overwinter in the adult 
stage, emerging from hibernation h\ the spring, and thus offer a more 
definite possibility of overwmtering the disease than the aphis, so 
that the later experiments have all been with the Diabrotica species. 

Artificial liibernation experiments. — ^Tho work with beetles was done 
by the wTiter m oooi3eration with Mr. Neale F. Howard, of the 
Bureau of Entomology, United States Department of Agriculture. 
Mr. Howard took entire charge of the collection and caging of the 
beetles and of all work pertainmg to the actual hibernation and life 
history of the hisects, and the writer conducted the tests relative 
to disease transmission by these insects. 

In the fall of 1916 some 8,000 beetles, chiefly Diahrotica viitata, 
were collected from fields about Madison, Wis., where mosaic was 
prevalent. The beetles were placed in hibernation cages of various 
sizes and types (PL IX, B) and fed on mosaic cucumber vines and 
fruits as long as such material was available. Most of the cages 
were kept outdoors in various places during the winter, a few being 
in more sheltered locations than others. The base of each cage 
contained from 1 to 3 feet of soil and trash, to furnish favorable 
conditions for hibernation. The number of insects varied with the 
size of the cage, but at least 200 were placed in each cage. During 
the winter some of the smaller cages were brought into the green- 
house at intervals and observed for the possible appearance of the 
beetles. Only thi-ee emerged dming the winter, however, and no 
infection resulted when they were placed on healthy cucmnber 
plants. Aside from these few insects, none appeared from any of 
the cages either in the early spring or in Jmie and July, so that 
no data on beetle overwintering of the disease was obtained from 
these tests. 

During the faU of 1917 these tests were repeated at Madison, Wis., 
on a larger scale, about 35,000 insects, both Diahrotica vittata and 
Diahrotica duodecimpunctata, being used. The beetles were collected 
during September and October and placed in various outdoor loca- 
tions, most of the cages being near points where beetles had appeared 
early that spring. Fifteen cages were used in these experiments, 
each having from 500 to 5,000 insects, most of them containing 
from 2,000 to 3,000. The cages were usually made with the base 
extending deeply into the soil but open at the bottom; the soil 



58 BUIXETIN 870, U. S. DEPARTMENT OF AGRICULTURE. 

within the cage was not disturbed and the natural con ering of grass 
and loaves was loft intact. The insects wore fed on the fruits and 
loaves of mosaic cucumber ]>lants as long as those could be secured. 
Later in the fall seeds of the wild cucumber {M'lcram pells lohata) 
wore scattered in all the cages, in order to furnish ])lants for an 
immediate tost of the beetles when they emerged in the si>riiig. 

On June 17, 1918, two beetles api)eared in each of two cages, 
but no others wore found in any cages during the rest of the season. 
The beetles which a])j>oared wore allowed to food on the wild cucum- 
ber plants present in the cages, but were not othei-wise tested. The 
writer did not obser\'e these plants after June 22, but other observers 
roport that no mosaic appeared. So far, therefore, the tests of booties 
liibernated under artificial conditions have given no evidence that the 
insects are concerned in the overwintering of the disease. 

Natural liihernation expenment<i. — Frequent obsei'vations were made 
in the fields at ^[adison, Wis., during the spring of 1917, and the first 
insects which appeared were collected and tested for mosaic infection. 
The fii"st beetles were noted on May 25, the insects suddenly ap])earing 
on seedlings of Micmmpelis lohata growing in a sand pit about 50 
yards from the main experimental plats of 1916 and 1917. About 
250 of these beetles were collected by the writer between May 25 
and Jmie 5, and varying numbers of them were placed on healthy 
cucumber plants under cages in the greenhouse. In this work 83 
plants were used, and the insects wore allowed to remain on them 
for two weeks, but no mosaic infection occurred. In addition to 
those tests, 60 small cucumber i)lants were sot out in the sand pit 
and in adjacent iields at i)oints where they would be likely to attract 
the beetles. Nearly all those i)lants were somewhat injured by the 
insects, but all remained healthy mitil after mosaic had dcvelojiod at 
other points in the locality. 

The first beetles to appear in the spring of 1918 were again tested 
in the same manner as in 1917. About 400 stripe<l beetles were 
collected between May 15, when they first appeared, and May 28. 
These were placed under cages with healthy cucumber i)lants in the 
greenhouse, 98 i)lants being used for the tests. About 100 beetles 
were also collected at Plymouth, Ind., during the latter part of May 
anil similarly tested on 25 healthy cucmnber plants. No mosaic 
ai)peared on any of these i)lants, however, although they were 
ol)serv'ed for tlireo weeks. Young ])lants were set out in the sand 
pit as in 1917, but no signs of mosaic developed, although the plants 
were exposed to the beetles from May 30 to Jmic 20. 

Field ohservations in 1917. — The striped beetles were very numerous 
about Madison, Wis., during June and July, 1917, and the young 
cucumbor plants, which aj)poarod about .lune 25 on the experimental 
plats, were severely injured by them. No mosaic was found in any . 



Jt! 



THE MOSAIC DISEASE OF CUCURBITS. 59 

field until July 19, althougli all plats were carefully inspected at 
intervals of two to four days. On July 19 two plats out of six 
within a radius of a mile developed a few cases of mosaic, and six 
days later 25 to 30 per cent of the plants in both })lats showed 
symptoms of the disease. The remaining fields also showed from 
5 to 15 per cent of mosaic plants on that date. All cases were of 
the same age and seemed to have developed almost simultaneously. 
In view of the fact that insects had been attacking the plants for 
four weeks previous to the appearance of the disease, while the 
period of incubation is normally 7 to 12 days, it is difficult to explain 
such a sudden and extensive outbreak of the disease on the basis of 
insect overwintering and transmission. 

That seedling plants may be infected when small but show no 
signs of the disease until they reach a certain period in their devel- 
opment has been considered a possible means of connecting such 
sudden outbreaks of the disease ^\'ith insect agencies. No such 
abnormally long incubation period has ever been noted, although 
at least 200 seedlings have been kept under observation for several 
weeks after inoculation. Secondary inoculations from such plants 
have never produced the disease except in cases where there were 
definite symptoms of mosaic in the original plant. There is no 
evidence, therefore, that such a prolonged incubation period does 
occm*. 

Further evidence against the theory that insects carry the disease 
over winter is also furnished by the results of the work on disease 
transmission during 1917. As shown in Table XVIII, the beetles 
collected after frost had killed the vines failed to induce the disease 
when placed in cages with healthy plants, while all beetles collected 
earlier in the season contained some individuals which were caiTiers 
of mosaic. It is quite probable, therefore, that only a very small 
percentage, if any, of the beetles which go into hibernation carry 
the disease, since they are present in the fields for some time after 
all mosaic vines have been killed by frost. In view of the results 
obtained in the artificial hibernation studies, it is likely that only a 
few of those beetles which go into hibernation survive the winter, 
and field observations also indicate that only a small percentage of 
the insects present in the fields in the fall appear in the spring. If 
this is the case, a large number of insects would necessarily have to 
be tested to insure finding individuals which carry the disease, if 
ever any such are present. Rand (23) has found that only a small 
number of the striped beetles which survive the winter are carriers 
of Bacillus tracheiphilus, and this is even more likely to be the case 
with cucumber mosaic. The number of beetles so far tested is too 
few, perhaps, to warrant definite conclusions, but the evidence to 
date does not lend much support to the theory that insects are 
instrumental in carrying the disease over winter. 



60 



BULLETIN S":?), U. S. DEPARTMENT OF AGRICULTURE. 



MICRAMPELIS LOBATA AS A SOURCK OF EARLY INFECTION. 

As 1ms alrondy ])ecn stated, there nre two wild species of the Cu- 
curbitacea? which arc native in the Middle West. Tliese are Mi- 
craiyipelis lohata and Sicyos angulutna. The latter does not occur as 
commonly in Micliigan and Wisconsin as M'lrrampdla lohata, "which 
is found in many localities where cucumbers are grown, cither wild 
or used as an ornamental vine. Observations in Michigan showed 
that the micrampelis was subject to a mosaic disease similar to that 
on cucumber, cases being observed during 1915 and 19 10 by Dr. 
E. A. Bessey, Mr. W. W. Gilbert, and the Avriter. Inoculations' 
made from mosaic cucumber to healthy ])lants of micrampelis in thel 
spring of 1917 resulted in typical cases of mosaic, and reciprocal, 
inoculations from mosaic micrampelis to cucumber were equally" 
successful, as shown in Table XX, thiis proving the identity of the 
two diseases. 

Tablf. XX. — Results of cross-inoculations from mosaic cucumber plants to healthy plants 
of Micrampelis lohata and from viosaic miirampelis to healthy nin/mhtr plants. 

INOCULATION.S FROM Mo.SAIC CUCUMBER I'LANTS TO HEALTHY PLANTS OF MICRAMPELIS. 





Treatment. 


Inoculum. 


Source of infection. 


Num- 
ber of 
plants 
inocu- 
lated. 


Results. 


Rate inocu- 
lated. 


Num- 
ber of 
mosaic 
plants. 


Date ob- 
served. 


May n, 1917.. 
Po 


Inoculated... 

Control 

Inoculated . . . 

Control 

Inoculated . . . 
Control 


Cru.shed ti.s.sue of mosaic 

leaf. 
Crashed ti.ssue of health v 

leaf. 
Crushed ti.ssue of mosaic 

leaf. 
Crushed ti.ssue of hcalthv 

leaf. 
Crushed ti.ssue of mosaic 

leaf. 
Crashed ti.ssue of hcalthv 

leaf. 
Crushed tissue of mosaic 

leaf. 
Crushed tissue of healthy 

leaf. 
Crashed ti.ssue of mosaic 

leaf. 
do 


Artificial infection. 


4 
4 
5 

5 
9 

9 
4 
4 

1 

6 
6 


1 



2 



7 



2 



1 

4 



May 22,1917 
Do. 


May 10,1017 
Do 


Artificial infection. 


May 29,1917 
Do. 


May :m,1917 
Do 


Artificial infection. 


June 10,1917 
Do. 


June 20,1017 ' Inoculated... 
Do ' Control 


Artificial infection. 


July 3, 1917 
Do. 


Auk. 31,1917 

Sept. 3,1917 
Do 


Inoculated... 
do 


Natural infection.. 
do 


Sept. 10,1017 

Sept. 12.1917 
Do. 


Control 


Crashed tissue of healtbv 
leaf. 









Inoculations from Mosaic Michampeus Plants to Hbaltuy Cttcumber Plants. 



May 24,1917 I Inoculated. 

Do ' Control 

July .1,1017 i Inoculated. 

Do 

Oct. S, 1917 

Do 

Oct. 30,1917 

Do 



Control 

Inoculated . . . 

Control 

Inoculated . . . 
Control 



Cnished ti.s.sue of mosaic 
leaf. 

Crashed ti-ssue of healthy 

leaf. 
Crushed ti-ssue of mosaic 

leaf. 
Crashed ti.ssue of healthv 

leaf. 
Crushed tissue of mosaic 

loaf. 
Crushed tLssuo of healthv 

leaf. 
Exprc«.sod Juico of mo 

snir pliuit. 
Expr(¥vs(><l Juice of 

hcalUiy plant. 



Inoculated from 
mosaic cucum- 
ber plant. 

Natural infection.. 



Natural infection. 



Natural infection. 



10 


9 


10 





8 


6 


7 





« 


2 








10 


7 


8 






June 9,1017 

Do. 
July 16,1917 

Do. 
Oct. 14,1917 
Oct. 20,1917 
Nov. 10,1917 

Do. 



THE MOSAIC DISEASE OF CUCURBITS. 61 

The fact that the micrampelis plant is so common, coupled with 
the presence of occasional mosaic infection, made it seem possible 
that it might be a source of spring infection to cucumber. During 
1917 a close inspection was made of all plants which could be found 
in the western portion of Madison, Wis., and in the country adjoining 
the fields where the cucumber plats were located. A particularly large 
number of micrampelis plants were found in the sand pit already 
referred to near the main experimental field. On July 9 eight 
plants in two groups were foimd at tliis point, all of them showing 
typical mosaic symptoms. No mosaic was found on any other 
cucurbit host until at least 10 days later. 

Mr. I. C. Hoffman, working at Plymouth, Ind., found several wild 
miarampelis plants which showed symptoms of mosaic as early as 
June 30. The plants in both cases were proved to be truly mosaic 
by successful inoculations on cucumber. 

No other cases were found prior to the appearance of the mosaic 
disease on the cultivated host in the spring of 1917, but during the 
summer 10 different centers of mosaic micrampelis were found in the 
vicinity of Madison at rather widely separated points. In the fall of 
1917 the disease was also reported by ^Ir. W. W. Gilbert as occurring 
at Big Kapids, Mch., and Brighton, Colo,, and by Mr. I. C. Hoffman 
at Fort Collins, Colo. 

Observations were again made about Madison during the spring 
of 1918, particularly in localities where the disease had appeared on 
micrampelis in 1917. In two suchlocations the disease was againfound 
on June 15. In one case 4 out of 18 plants were infected with mosaic 
and in the other case 6 out of 27 showed it. In all cases the plants 
were about 5 weeks old. Cultivated cucurbits were being planted at 
that time, and none of the adjacent gardens contained any cucurbits 
of other species. Several mosaic micrampelis vines were also found 
by Mr. W. W. Gilbert at Big Rapids, Mich., on June 23, 1918. These 
plants were being gi'own as ornamentals near a garden where the 
disease had been found the previous season on cucumbers. They 
were 4 to 6 feet tall when seen, while the cucumbei-s in near-by gar- 
dens were only a few inches high and in the adjacent fields few were 
up. A considerable number of striped beetles were observed feeding 
on the micrampelis vines. 

There is no doubt, therefore, that the disease occurs on wild plants 
some time before its appearance on cultivated hosts, since most 
cucumber fields had just been planted at the time the disease ap- 
peared on the micrampelis vines. The source of infection on the 
micrampelis plants in these cases was undetermined. To test the 
possibility of seed overwintering, about 150 plants were grown from 
seed collected in each of the Madison localities in 1917. The plants 
were grown in the greenhouse for six weeks during January and Feb- 



62 BULUiTlN 879, U. S. DEPARTMENT OF AGRICULTURE. 

ruiirv, 1918, in comuM-tioM witli tho othor test of sood fn>in mosaic 
micninipolis montioiuul l)olo\v. No mosaic syinploins apjwarod on. 
anv of tho plants. No mosaic was present in the greenhouses in the' 
\'icinity of Madison, and while striped ])Ootles attacked the vines,, 
their importiince in overwintering^ the disease is unknown. 

Wild micrampelis i)lants arc common in all parts of Michii;an and 
Wisconsin, and in addition to their occurrence in nature the vine is 
often planted whore a quick-j^-owincr ornamental is desired. After 
being once plant-od it usually continues to appear each year from _ 
self-sown seed. | 

The presence of mosaic on Micrampelis lohata in advance of its 
appearance on any cultivated cucurbit host, coupled with the fact 
that the striped beetles feed upon micrampelis as soon as they emerge! 
from liibernation and go directly from the wild plant to the cultivatedi 
cucumber when the latter appears, oll'ci-s a most promising cxplana-' 
tion for the early infections of the cucumber. There is little doubt, 
apparently, that Micrampelis lohata may be a source of such early 
infection to the cucumber. 

The source of primary infection on the wild plant is still uncertain, 
but much of the work on the overwintering of tho disease on the cul-. 
tivated cucumber seems equally applical^lo to tho wild species. Soil 
is probably not a factor and the question of seed as a means of trans- 
mission is still unsettled, although the appearance of the disease in 
tho same spot each year would indicate that tho seed may bo the 
means of overwintering. During the winter of 1917-18, 1,100 plants 
were gi'own in the greenhouse from seed collected from mosaic 
micrampelis plants in Michigan, Indiana, Wisconsin, and Colorado. 
An additional lot of 1 ,000 plants was also gi-own from seed taken from 
plants supposedly free from mosaic. The conditions under which the 
plants were gi-own were unfavorable, however, and caused the leaves 
to develop various abnormal colorings, which made it difficult to 
determine whether any of the plants were infected with mosaic. 
Inoculations from suspected plants gave negative results in all cases, 
but further tests of seed from mosaic plants are in progress under 
more favorable conditions for observation.* 

WILD NONCIICURBITACEOIIS HOSTS. 

Tho work of Jaggor (19), already mentioned, has shown that the 
mosaic disease of cucurbits may be transfoired to some plants outside 
the Cucurbitacetr. Although thoie is no definite evidence that the 
disease overwinters on plants of other familiivs, it is possible that such 
wild hosts, particularly if they are perennial, might serve as sources 
of early infection to the cucurbits, especially as cucum)>er insects are 
often found feeding on plants of other families. AU cross inocula- 

> Subsequent tc.its hnvo proved that the mosaic disoasc may bo carried over winter lntboae«d<l( 
Micramjiflit lobata. Soo rhytopatholoffy, vol. 0, p. 32(>, 1919. 



THE MOSAIC DISEASE OF CUCURBITS. 63 

tions made by the wi'iter from plants outside the Cucurbitacese have 

given negative results, but the large number of wild plants which 

show various types of chlorotic diseases makes the problem one which 

will require much inoculation work l)efore final conclusions are 

justified. 

CONTROL MEASURES. 

All efforts to control the mosaic of cucurbits have dealt wnith 
cucumbers in the field and have been along three lines: (1) The 
removal of diseased plants as soon as they are detected, (2) the 
control of insects which spread the disease, and (3) the breeding of 
a cucumber which would be resistant to the mosaic disease. 

SANITARY MEASURES. 

The tests of sanitary measures were made during the season of 
1916, a considerable acreage of land being available for this pur- 
pose at Plymouth, Ind., Madison, Wis., and Big Rapids, Mich. The 
absence of the disease from all but one field at Big Rapids limited 
the work to a single plat of about one-third of an acre, where aphids 
from mosaic experiments escaped by accident and furnished a 
severe infection. The disease appeared in the outside row of the 
plat on July 28, the aphids being by that time well disseminated 
on the rows closely adjoining. All plants in this row were pulled 
and burned where they lay, the remainder of the field was sprayed 
with nicotine sulphate, and from that date to the end of the season 
plants in the plat were inspected daily and all those suspected of 
mosaic were removed and destroyed. Once started, however, the 
disease continued to increase, and by August 25 the number of 
plants had been reduced about 50 per cent. The removal of dis- 
eased vines after they had become intertwined with healthy plants 
resulted in contact infection of those adjacent and served to increase 
rather than diminish the disease. 

The work at Madison, Wis., was under the direction of Dr. M. W. 
Gardner during 1916 and included a large acreage of cucumbers. 
The disease developed rapidly after its first appearance, due prob- 
ably to the presence of an abnormal number of cucumber insects, 
and attempts at removing all mosaic plants soon become hopeless, 
many fields being practically ruined by August 9. Similar results 
were secured in the experiments at Plymouth, Ind., which were 
conducted by Dr. George A. Osner. 

The fact that diseased plants are a source of infection before 
definite symptoms appear, which had not been proved at that time, 
adds to the difficulty of control by sanitary measures. While the 
eradication method is theoretically sound, the practical conditions in 
the field are such that, except for use early in the season and in 
the case of slight infections in isolated fields, this method affords 
little hope of success in stopping the progress of the disease. 



( 



I 



64 BUT. T.F.TIN 879, U. S. DEPARTMENT OF AGRICULTURE. 

INSECT CONTROL. 

Tlio attempts at insect coiitiol have boeii urulor the <Iiro{'ti(m of 
Mr. Neale F. Howard, of the J3uroau of Kntomology. Tliis work 
was conducted at Madison, Wis., and Plymouth, Ind., and is still 
in progress. It is possible to control insects to such a degree as will 
prevent severe injury to the plants, but complete insect elimination 
is necessary to prevent disease transmission by them, and methods 
for accomplishing this have not yet been worked out. In the green- 
house, insect elimination is more easily possible, and if the grower 
is well acc[uainted with mosaic he might succeed in eradicating or 
at least so checking the disease that little loss would occur. In 
the field, however, some further knowledge of the method of over- 
wintering of the disease and the cause of its sudden appearance in 
the spring is necessary before adequate methods of control can be 
devised. 

RESISTANT VARIETIES. 

Attempts have been made to obtain a variety of cucumber re- 
sistant to the mosaic disease, but so far this work has been unsuc- 
cessful. Selections made in a mosaic field at Muscatine, Iowa, in i 
1915 by Mr. W. W. Gilbert were tested at Holland, Mich., in 1916, 
but the amount of infection present in the field was not sufTicient 
for a fair trial. Selections from this strain which gave some promise 
of resistance were again planted at Madison, Wis., in 1917, but all the 
plants developed mosaic symptoms early in the season. During the 
past four seasons the writer has never observed any indication of 
resistance in any variety of cucumbers, and the fact that little 
evidence of resistance has appeared in the case of other mosaic 
diseases indicates that the (levelo])ment of a resistant variety is 
likely to be difficult, if at all possible. 

SUMMARY. ill 

The mosaic disease of cucurbits has apparently been present in 
the United States for nearly 20 years, but prior to 1911 its im- 
portance was practically unrecognized. 

nie disease appears both in the field and in the greenhouse in 
nearly all sections where cucm-bits are of commercial importance. 
Nearly all cultivated cucurbits are susceptible to the disease, but 
the cucumber crop seems to be most seriously affected, particularly 
in the Central States and the trucking regions of the South. 

The diseased plants develop a yellow motthng of the younger 
leaves, accompanied by a wrinkled or savoyed appearance. The 
older leaves gradually turn yellow and die, leaving the basal por- 
tion of the stem bare. These bare stems terminate in a rosettelike 
cluster of dwarfed leaves, which lie close to the ground, owing to the 



THE MOSAIC DISEASE OF CUCURBITS. 65 

shortening of the petioles and internodes of the stem. Mosaic 
fruits of the cucumber are mottled with green and yellow and often 
develop dark-green wartlike outgrowths. The Summer Crookneck 
squash also shows a mottled and warted appearance, but the fruits 
of most other cucurbits are little changed. 

Nearly all species and varieties of the genera Cucumis, Cucurbita, 
Lagenaria, Luffa, Momordica, Trichosanthes, Ecballium, Benincasa, 
Micrampelis, and Sicyos are susceptible to the disease, but the 
Citrullus species seem to be partially resistant. 

The pathologic anatomy of the mosaic leaves shows a distinct 
variation from the normal in the tissues of the mottled leaves. The 
palisade cells of the green portions of such leaves are longer and 
narrower than similar cells in the yelloY portions. The spongy 
parenchyma is more compact in the yellowed areas, and the chloro- 
plasts are somewhat smaller than those of the green portions of the 
leaf. Similar differences appear in the cells directly below the epi- 
dermis in mottled cucumber fruits, but all the other tissues of the 
fruit are normal in appearance. The structure of the stems and roots 
of mosaic plants does not differ from that of healthy plants. 

No visible causal organism has been associated with cucurbit 
mosaic, and the disease appears to be unrelated to soil conditions. 
The juice of mosaic plants contains an infective principle, or virus, 
however, which possesses certain definite properties. 

The expressed juice of mosaic plants is rendered noninfectious if 
heated above 70° C. The power of infection is also destroyed by 
formaldehyde, phenol, and copper sulphate in 0.5 per cent solutions 
and by mercuric chlorid in a strength of 1:2,000. A 10 per cent 
solution of chloroform will also render the virus inactive, but neither 
5 per cent chloroform nor 10 per cent toluene are effective. 

The juice of mosaic diseased plants maybe diluted to 1:10,000 and 
still retain the power of infection. Filtration of the expressed juice 
of mosaic plants through a Berkefeld filter does not remove this 
power of infection, but Chamberland filters have rendered the filtrate 
noninfectious. The expressed juice of mosaic plants rarely remains 
infectious longer than 24 to 48 hours, and the virus is rapidly destroj'-ed 
by desiccation. 

The infective principle, as far as it has been determined, possesses 
many properties of a living organism, and it appears possible that the 
disease may be caused by an ultramicroscopic parasite. 

The mosaic is highly infectious and can be produced by intro- 
ducing the expressed juices or crushed tissues of a mosaic plant into 
slight wounds in healthy plants. Inoculations may be made at any 
point in the stem or leaf, including the leaf trichomes. Infection 
can also be produced through the fruit, but has never resulted when 
185lis°— 20 5 



GO BULL.KTIN 87!), U. S. DEPARTMENT OF AGRICULTtTRK. 

tho virus was introduced into tlio roots or flower parts. The first 
symptoms of the disoaso invariably appear in the younji^ost leaves or 
fruits and the susceptihilit}' of the plant seems to be closely related 
to its a^e and vigor of growth. 

The virus spreads through the pbmt from the point of inoculation 
and is present throughout the leaves and stems 24 to 48 hours before 
any visible symptoms appear. There is some evidence that the vas- I 
cular sj^stem may be the chief channel of distribution in the plant. 

Transmission under field conditions in the case of the cucumber 

may occur during the thinning, training, or picking operations and 

also by the removal of diseased vines whidi are intertwined with 

healthy plants. Cucumber insects are among tho most important 

asrents in the transmission of the disease in both the field and tho 

* 
greenhouse. The melon aphis (Aphis gossypii Glover) and the striped 

and r2-spotted cucumber beetles (Diahrotica vittata Fabr. and Dia- 

hrotica duodecimpunctata Oliv.) are the insects most concerned. Bees 

have not been proved to be carriers of the disease. 

It has been showni that the disease does not live overwinter in the 
soil, and there is no evidence that striped cucumber beetles or other 
insects are a source of primary infection in the spring. Extensive 
field tests of seed from mosaic plants and observations in the field 
and greenhouse indicate that infection through the seed may possi- 
bly occur in rare cases. 

The mosaic diseases of tobacco, tomato, bean, potato, pokeweed, 
and various other hosts do not appear to affect the cucumber. Inocu- 
latioiis of these and other i)lants outside the Cucurbitacea' (except 
martynia, see p. 0) with the exi)rcssod juice of mosaic cucurbits have 
also given negative results. 

The wild cucunilx'r {Micranipdis lohata) is affected with a mosaic 
disease identical with that on the cucumber. Diseased i)lants of this 
species have been found in Wisconsin and Indiana at least two or 
more weeks before the disease appeared on the cultivated cucumber. 
The striped cucumber beetles feed on the micrampelis from the time 
they appear and later go directly from the wild plants to cultivated 
cucumbers. There is thus a direct raeans of transmission from the 
wild to the cultivated host during the early part of the season. 

The source of primary infection of micrampelis is not certain, the 
factors which appear to have l)een eliminated in the case of the cul- 
tivated cucumber j)robably being excluded from the ^^^ld host also. 
The wild host plant, however, offers the most definite source yet dis- 
covered of primary infection for the cucumlier. 

The control methods thus far tested have been (1) the removal of 
diseased plants as soon as found; (2) the control of cucumber insects, 
which are largely instrumental in sj)reading the disease; {'A) the dis- 
covery of varieties of cucumbers which are resistant to mosaic; and 



THE MOSAIC DISEASE OF CUCURBITS. 67 

(4) the caging of plants to keep insects away. None of these methods 
has proved entirely effective and practicable under field conditions. 

In the field, removal of diseased plants has proved of little value 
except for the first cases early in the season, because they may be 
sources of infection for one to three daj's before they are found. 
Even with the best insect-control methods available, enough beetles 
are left to cause a wide dissemination of mosaic. All efforts at find- 
ing strains of cucumbers resistant to the disease have given negative 
results. The use of cages is practicable for a few plants in the 
home garden, but not feasible on larger areas. 

Under greenhouse conditions the elimination of insects and the 
removal of mosaic plants are possible, and these control measures 
have proved of great value. 

Satisfactory control measures for field conditions will necessarily 
have to await the discovery of more definite data regarding the 
sources of primary infection. 

The importance of wild host plants as a means of overwintering 
the mosaic disease, the possibilities of infection through seeds from 
diseased plants, and the relation of insects to overwintering must be 
studied further before effective control recommendations can be made. 



LITERATURE CITED. 

Allaku, II. A. 

(1) 1914. The mosaic disease of tobacco. U. S. Dept. Agr. Bui. 40, 33 p., 7 pi. 

(2) 1915. Effect of dilution on the iiifectivity of the virus of the mosaic disease 

of tobacco. 7u Jour. A},'r. ReHcarch, v. 3, no. 4, p. 295-299. 

(3) 1915. DLstribution of the virus of the mo.saic disease in capsules, filaments, 

anthers, and pistils of affected tobac<o plants. In Jour. Ajjr. Re- 
search, V. 5, no. 6, p. 251-256, pi. 23. 

(4) 1916. Some properties of the virus of the mosaic disease of tobac<o. 7« Jour. 

Apr. Research, v. G, no. 17, p. 649-674, pi. 91. Literature cited, 
p. 673-674. 

(5) 1917. Further studies of the mosaic disease of tobacco. In Jour. Apr. Re- 

search, V. 10, no. 12, p. 615-{)32, pi. 63. 

(6) Beuerinck, M. W. 

1898. Ueber ein Contapium vivum fluidum als Ursache der Fleckenkrankheit 
der Tabak.sbliitter. Verhandel. K. Akad. Wetensch. Amsterdam, 
sect. 2, deel 6, no. 5, 22 p., 2 col. pi. 

(7) Chapm.\n, G. it. 

1917. Masaic disease of tobacco. Mass. Agr. Exp. Rta. Bui. 175, p. 73-117, 

5 pi. 
Clinton, G. P. 

(8) 1908. Notes on fungus diseases etc., for 1908. Muskmelon chlorosLs. In 

Conn. Agr. Exp. Sta. Ann. Rpt. [1906]/08, p. 865-866. 

(9) 1916. Mosaic, or white pickle. In Conn. Agr. Exp. Sta. Ann. Rpt. 1915, p. 

430-431, pi. 19. 

(10) Coons, G. n. 

1915. Notes on plant diseases of Michigan. Cucumber mosaic, or white 

pickle. 17th Rept. Mich. Acad. Science., p. 125-126, 1 i>l. 

DOOLITTLE, S. P. 

(11) 1916. A new infectious mosaic disease of cucumber. In Phytopathology, 

V. 6, no. 2, p. 145-147. 

(12) and Gilbert, W. W. 

1918. Further notes on cucumber mosaic disease. (Abstract.) In Phyto- 

pathology, V. 8, no. 2, p. 77-78. 

(13) Freibero, G. W. 

1917. Studies in the mosaic diseases of plant.';. In Ann. Mo. Bot. Gard., 
V. 4, p. 175-232, pi. 14-17. l$ibliograi)hy, j). 223-225. 

(14) Gilbert, W. W. 

1916. Cucumber mosaic disease. In Phytopathology, v. 6, no. 2, p. 143- 

144, pi. 5. 

(15) IIUNOEK, F. W. T. 

I'tO.'). Untcrsiichungcn nnd Betrachtungon iihcr die Mosaikkrankheit der 
Tabak.spflanze. In Ztschr. Pllanzenkrank., Bd. 1"), Heft 5. p. 257- 
_ 311, 1 fig. Lit^ratur, p. 309-311. 

(16) IWANOWSKI, I). 

1903. librr die Mo.saikkrankheit der Tabaksi)flan/.p. Tn Zts. hr. Pflanzen- 
krank., Bd. 13, Heft 1, p. 1-41, pi. 1-3. 
68 



THE MOSAIC DISEASE OF CUCURBITS. 69 

Jagger, I. 0. 

(17) 1916. Experiments with the cucumber mosaic disease. In Phytopathology, 

V. 6, no. 2, p. 148-151. 

(18) 1917. Two transmissible mosaic diseases of cucumber. (Abstract.) J/i Phy- 

topathology, V. 7, no. 1, p. 61. 

(19) 1918. Hosts of the white pickle mosaic disease of cucujgber. In Phytopath- 

ology, V. 8, no. 1, p. 32-33. 

(20) 1918. Mosaic disease of cucurbits. (Abstract.) In Phytopathology, v. 8, 

no. 2, p. 74-75. 

(21) KONING, C. J. 

1899. Die Flecken- oder Mosaikkrankheit des hollandischen Tabaks. In 
Ztschr. Pflanzenkrank., Bd. 9, Heft 2, p. 65-80, 2 fig. 

(22) McClintock, J. A. 

1916. Is cucumber mosaic carried by seed? 7/i Science, n. s., v. 44, no. 
1144, p. 786-787. 

(23) Rand, Frederick V. 

1915. Dissemination of bacterial wilt of cucurbits. (Preliminary note.) In 

Jour. Agr. Research, v. 5, no. 6, p. 257-260, pi. 24. 

(24) Reddick, Donald, and Stewart, V. B. 

1918. Varieties of beans susceptible to mosaic. In Phytopathology, v. 8, 
no. 10, p. 530-534. 

(25) RuGGLES, A. G., and Stakman, E. C. 

1911. Orchard and garden spraying. Minn. Agr. Exp. Sta. Bui. 121, 32 p. 
Selby, a. D. 

(26) 1903. Report of committee on vegetable pathology. A peculiar malady of 

forced cucumbers. In Ann. Rpt. Ohio State Hort. Soc, 1902, p. 109. 

(27) 1910. A brief handbook of the diseases of cultivated plants in Ohio. Ohio 

Agr. Exp. Sta. Bui. 214, p. 307^56, 106 fig. Literature, vii p. 

(28) Stakman, E. C, and Tolaas, A. G. 

1916. Fruit and vegetable diseases and their control. Minn. Agr. Exp. Sta. 

Bui. 153, 67 p., 32 fig. 

(29) Stone, G. E. 

1910. Calico or mosaic disease of cucumber and melon. In Mass. Agr. Exp. 
Sta. 22d Ann. Rpt. [1909], pt. 1, p. 163. 

(30) Westerdijk, Johanna. 

1910. Die Mosaikkrankheit der Tomaten. Meded. Phytopath. Lab. "Willie 
Commelin Scholten," Amsterdam, 1, 19 p., 3 pi. (1 col.). 
Woods, A. F. 

(31) 1900. Stigmonose. A disease of carnations and other pinks. U. S. Dept. 

Agr., Div. Veg. Physiol." and Path. Bui. 19, 30 p., 5 fig., 3 pi. 
(1 col.). 

(32) 1902. Observations on the mosaic disease of tobacco. U. S. Dept. Agr., Bur. 

Plant Indus. Bui. 18, 24 p., 6 pi. (3 col.). 



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